feat: add ATLASSIAN_* env var fallbacks for shared credentials

[rianjs]

Summary

• Add ATLASSIAN_URL, ATLASSIAN_EMAIL, ATLASSIAN_API_TOKEN env var fallbacks
• Enables shared credentials between cfl (Confluence) and jtk (Jira) CLIs
• Tool-specific vars (CFL_*) take precedence over shared vars (ATLASSIAN_*)

Precedence Order

Setting	Lookup Order
URL	CFL_URL → ATLASSIAN_URL → config
Email	CFL_EMAIL → ATLASSIAN_EMAIL → config
Token	CFL_API_TOKEN → ATLASSIAN_API_TOKEN → config

Usage

Shared credentials (both tools):

export ATLASSIAN_URL=https://mycompany.atlassian.net
export ATLASSIAN_EMAIL=user@example.com
export ATLASSIAN_API_TOKEN=your-token

Per-tool override:

export ATLASSIAN_EMAIL=user@example.com
export ATLASSIAN_API_TOKEN=your-token
export CFL_URL=https://confluence.internal.corp.com  # Different URL for Confluence only

Changes

• internal/config/config.go: Add getEnvWithFallback() helper, update LoadFromEnv()
• internal/config/config_test.go: Add tests for fallback behavior and precedence
• README.md, CLAUDE.md: Document new env vars

Test Plan

• Build passes
• All tests pass (including new ATLASSIAN_* tests)
• Lint passes

Closes #99

[rianjs]

Test Coverage Assessment

Summary

The test coverage for this PR is excellent. The new functionality is well-tested with appropriate coverage of all critical paths.

What's Tested

getEnvWithFallback helper function:

• Returns primary value when set
• Falls back when primary is empty
• Returns empty when both are empty

LoadFromEnv with ATLASSIAN_ fallbacks:*

• ATLASSIAN_* vars used when CFL_* not set (URL, email, token)
• CFL_* takes precedence when both are set
• Mixed usage (e.g., CFL_URL with ATLASSIAN_EMAIL/TOKEN)

Assessment

Aspect	Status	Notes
Core logic tested	Covered	All three fallback scenarios covered
Edge cases	Covered	Empty vars, mixed vars, precedence
Integration with existing tests	Good	Existing TestConfig_LoadFromEnv tests still pass and complement new tests

No Gaps Identified

The PR adds:

1. A simple 5-line helper function (getEnvWithFallback)
2. Updates to LoadFromEnv to use the helper for 3 config fields

Both are fully covered by the new tests. The test structure follows existing patterns in the codebase and properly cleans up environment variables.

Verdict: No additional test coverage needed. The tests verify the documented behavior (precedence order) and cover the realistic use cases described in the PR.