Update CONTRIBUTING.md project structure diagram

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Task summary

CONTRIBUTING.md lists git/ as only containing diff.ts, but the repo also has git/hook.ts. It also omits commands/completion.ts and commands/config.ts. Update the project structure diagram to match the current codebase.

Why this is a good first issue

Low-risk documentation fix. A great first PR to learn the repo layout before touching code.

Suggested files or areas

• CONTRIBUTING.md — project structure section
• AGENTS.md — more complete and up-to-date reference
• src/git/hook.ts, src/commands/completion.ts, src/commands/config.ts

Acceptance criteria

• git/ section includes hook.ts
• commands/ section includes completion.ts and config.ts
• Structure diagram matches actual src/ layout

Extra context

AGENTS.md has a more complete architecture diagram — use it as the source of truth.