feat: add write capability to TrailerService (GIT-66)

[TonyCasey]

Summary

• Adds addTrailers(trailers, cwd) method — amends HEAD commit to append AI-* trailers, deduplicating against existing trailers
• Adds buildCommitMessage(message, trailers) method — pure string builder for composing commit messages with a trailer block
• Detects existing trailer blocks (lines after last blank line matching Key: Value) to avoid double blank lines
• 11 new tests covering both methods and edge cases

Depends on #34 (GIT-65)
Closes GIT-66

Test plan

• npm run pre-commit passes (type-check + lint)
• All 281 unit tests pass (11 new)
• addTrailers amends HEAD correctly, preserves existing trailers, deduplicates
• buildCommitMessage handles: no trailers, existing trailers, body text, trailing whitespace
• Empty trailers array is a no-op (SHA unchanged)

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Append trailers to commit messages with duplicate prevention and preservation of existing trailers; builds properly formatted commit messages.
• Tests
  • Added extensive tests covering trailer formatting, message-building scenarios, and commit amend behavior to ensure reliability.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Introduces a TrailerService and adds two ITrailerService methods: addTrailers(trailers, cwd?) to amend HEAD with AI trailers (preserving existing and skipping duplicates) and buildCommitMessage(message, trailers) to produce a message string with appended trailers. Registers service in DI and adds unit tests.

Changes

Cohort / File(s)	Summary
Interface src/domain/interfaces/ITrailerService.ts	Added two method signatures: addTrailers(trailers: readonly ITrailer[], cwd?: string): void and buildCommitMessage(message: string, trailers: readonly ITrailer[]): string.
DI container / types src/infrastructure/di/container.ts, src/infrastructure/di/types.ts	Imported and registered TrailerService as a singleton and exposed trailerService: ITrailerService on the container cradle.
Service implementation src/infrastructure/services/TrailerService.ts	Implemented addTrailers() (reads existing trailers, filters duplicates, reads HEAD message, builds amended message, and amends HEAD) and buildCommitMessage() with a private hasTrailerBlock() helper to detect/append trailer blocks.
Tests tests/unit/infrastructure/di/container.test.ts, tests/unit/infrastructure/services/TrailerService.test.ts	Added tests verifying DI exposure/singleton and comprehensive unit tests for buildCommitMessage() edge cases and addTrailers() behavior against a temporary git repo (preserve existing, avoid duplicates, add multiple, no-op on empty).

Sequence Diagram

sequenceDiagram
    participant Caller as Client
    participant TS as TrailerService
    participant Git as Git Repository

    Caller->>TS: addTrailers(newTrailers, cwd?)
    activate TS

    TS->>Git: query/read existing trailers (queryTrailers/readTrailers)
    Git-->>TS: existingTrailers

    TS->>TS: filterDuplicates(newTrailers, existingTrailers)
    TS->>Git: read HEAD commit message
    Git-->>TS: currentMessage

    TS->>TS: buildCommitMessage(currentMessage, filteredTrailers)
    TS->>TS: hasTrailerBlock(currentMessage)
    TS-->>TS: amendedMessage

    TS->>Git: amend HEAD with amendedMessage
    Git-->>TS: success

    deactivate TS
    TS-->>Caller: void

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Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

🚥 Pre-merge checks | ✅ 3 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'feat: add write capability to TrailerService' clearly and directly summarizes the main change—adding write functionality (addTrailers and buildCommitMessage methods) to the TrailerService.
Merge Conflict Detection	✅ Passed	✅ No merge conflicts detected when merging into main

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch git-66

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[Copilot]

Pull request overview

Adds write support to the existing TrailerService so the tool can append AI-* trailers to the current commit, and introduces a pure helper for composing commit messages with trailer blocks. This builds on the DI registration work from GIT-65 and expands unit coverage around trailer handling.

Changes:

• Add TrailerService.addTrailers() to amend HEAD and append (deduped) trailers.
• Add TrailerService.buildCommitMessage() + trailer-block detection to avoid double blank lines.
• Expand DI + unit tests for the new service capabilities.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/infrastructure/services/TrailerService.ts	Implements addTrailers and buildCommitMessage, plus trailer-block detection logic.
src/domain/interfaces/ITrailerService.ts	Extends the domain interface with the new write and message-building methods.
src/infrastructure/di/types.ts	Adds trailerService to the ICradle type.
src/infrastructure/di/container.ts	Registers TrailerService in the Awilix container as a singleton.
tests/unit/infrastructure/services/TrailerService.test.ts	Adds unit tests for buildCommitMessage and git-backed tests for addTrailers.
tests/unit/infrastructure/di/container.test.ts	Extends container tests to cover trailerService resolution.

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[Copilot]

addTrailers is documented as appending only AI-* trailers, but it currently accepts and appends any ITrailer keys. Since readTrailers() only returns AI-* trailers, passing non-AI trailers would bypass deduplication and can lead to duplicate non-AI trailers being appended on repeated calls. Consider filtering/validating inputs to AI_TRAILER_PREFIX (or isAiTrailer) and either ignoring or rejecting non-AI keys so behavior matches the docs and dedupe logic.

Suggested change

	// Read existing trailers to avoid duplicates
	const existing = this.readTrailers('HEAD', cwd);
	const existingKeys = new Set(existing.map(t => `${t.key}:${t.value}`));
	const newTrailers = trailers.filter(t => !existingKeys.has(`${t.key}:${t.value}`));
	// Only operate on AI-* trailers to match readTrailers/parseTrailerBlock behavior.
	const aiTrailers = trailers.filter(t => t.key.startsWith(AI_TRAILER_PREFIX));
	if (aiTrailers.length === 0) return;
	// Read existing trailers to avoid duplicates
	const existing = this.readTrailers('HEAD', cwd);
	const existingKeys = new Set(existing.map(t => `${t.key}:${t.value}`));
	const newTrailers = aiTrailers.filter(
	t => !existingKeys.has(`${t.key}:${t.value}`)
	);

[TonyCasey]

Fixed — added AI_TRAILER_PREFIX filter at the top of addTrailers so non-AI trailers are rejected before deduplication. See 8fcd93f.

[Copilot]

This test block claims to check the resolved TrailerService interface, but it only asserts the pre-existing methods. Since ITrailerService now includes addTrailers and buildCommitMessage, either add assertions for those methods too or rename the test to avoid going stale/misleading.

Suggested change

	assert.equal(typeof trailerService.queryTrailers, 'function');
	assert.equal(typeof trailerService.queryTrailers, 'function');
	assert.equal(typeof trailerService.addTrailers, 'function');
	assert.equal(typeof trailerService.buildCommitMessage, 'function');

[TonyCasey]

Fixed — added assertions for addTrailers and buildCommitMessage to the container interface test. See 8fcd93f.

[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@tests/unit/infrastructure/di/container.test.ts`:
- Around line 166-179: Update the DI unit test for trailerService to assert the
new methods on the resolved service: verify that container.cradle.trailerService
exposes addTrailers and buildCommitMessage as functions in the same style as
existing assertions for readTrailers, formatTrailers, and queryTrailers; keep
the existing singleton test unchanged so it still confirms
container.cradle.trailerService === container.cradle.trailerService.