feat: add optional semantic color to discovery output

[kioku]

Summary

• add a centralized DiscoveryStyle module for semantic ANSI styling
• apply optional color to human-facing discovery surfaces (overview, commands, docs, API landing, config api add, and search)
• gate color by stdout TTY and NO_COLOR, while keeping machine/JSON output uncolored
• add tests for colored rendering, NO_COLOR disablement, and ANSI-free structured output

Validation

• cargo test
• cargo fmt --all --check
• cargo clippy --all-targets --all-features -- -D warnings

Closes #175

[kioku]

Implementation plan used for this PR (/tmp/plans/aperture-175.md):

Plan

# Issue 175 Implementation Plan

## Summary
Add optional semantic ANSI color to human-facing discovery and onboarding output while keeping structured/machine output uncolored. Color should be centralized, TTY-aware, and disabled when `NO_COLOR` is set.

## Branch
`175/semantic-discovery-colors`

## Implementation Steps
1. Add a centralized discovery styling module (`src/discovery_style.rs`) with:
   - semantic style roles (API title, headings, metadata, next-step labels, HTTP methods, required markers, success/warning)
   - ANSI rendering helpers
   - enablement detection based on `stdout` TTY and `NO_COLOR`
   - unit tests for enablement and rendering behavior
2. Thread optional style support through discovery formatters:
   - update `src/docs.rs` to support styled rendering for overview/reference/docs/help list output
   - keep plain-text generation behavior available for non-colored paths/tests
   - ensure paths/payload-like text remain unstyled
3. Apply style usage in CLI command handlers for required surfaces:
   - `src/cli/commands/docs.rs` (`overview`, `commands`, `docs` text modes)
   - `src/cli/commands/api.rs` landing output next-step guidance
   - `src/cli/commands/config.rs` `config api add` success/partial acceptance/next-step labels
   - `src/search.rs` / `src/cli/commands/search.rs` minimal semantic styling for human-readable results
4. Add/adjust tests to satisfy acceptance criteria:
   - colored TTY case (formatter rendered with forced-enabled style)
   - `NO_COLOR` case (style detection test)
   - machine-output case (assert no ANSI escapes in JSON/structured output)
5. Run targeted tests and full quality gate (`cargo test && cargo fmt && cargo clippy`), then fix any regressions.

## Testing Approach
- Targeted unit/integration tests:
  - discovery style module tests for TTY and `NO_COLOR` behavior
  - docs/search formatting tests for styled output (escape sequences present when style enabled)
  - structured output tests ensuring JSON remains parseable and uncolored
- Full project validation:
  - `cargo test`
  - `cargo fmt --all --check`
  - `cargo clippy --all-targets --all-features -- -D warnings`

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 93.70861% with 19 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/cli/commands/docs.rs	26.08%	17 Missing ⚠️
src/docs.rs	98.57%	2 Missing ⚠️

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