feat: parsers, git walker, and matcher

[hermanngeorge15]

Summary

• GSD SUMMARY.md parser (8 tests)
• GSD REQUIREMENTS.md parser (6 tests)
• Git commit walker with tag listing (4 tests)
• Commit-to-phase matcher (6 tests)
• Test fixtures for GSD files

PR 2 of ai-changelog v0.0.1 milestone.

Changes

• src/parsers/gsd-summary.ts — extracts phase number, what was built, decisions, files changed, and spec requirements from GSD SUMMARY.md files
• src/parsers/requirements.ts — extracts phases with features and acceptance criteria from GSD REQUIREMENTS.md files
• src/git.ts — walks git commit history between refs using simple-git, parses conventional commits, lists tags
• src/matcher.ts — links commits to GSD phases by file path overlap (exact match + directory prefix fallback)
• Test fixtures: summary-phase1.md, summary-phase2.md, requirements.md

Test plan

• 32 total tests pass (8 commit parser + 8 GSD summary + 6 requirements + 4 git walker + 6 matcher)
• TDD: tests written before implementation for all 4 tasks
• Prettier formatting applied before each commit

Note on git walker

The plan's implementation passed from: undefined to simple-git's log(), which generates an invalid range (..HEAD) returning 0 results. Fixed by conditionally building log options — only including from/to when from is provided.

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