[ENHANCEMENT] Add Git context collector for commit-message generation #282

Problem (one or two sentences)

Zoo needs a safe, scriptable way to inspect Git changes before AI commit generation can work.

Part of #145.

Context (who is affected and when)

Developers may have staged, unstaged, renamed, copied, deleted, binary, or untracked files when preparing a commit from VS Code Source Control.

Desired behavior (conceptual, not technical)

Zoo can collect commit-relevant Git context without using Zoo file ignore filters, preferring staged changes and falling back to unstaged changes only when nothing is staged.

Constraints / preferences (optional)

Use stable, machine-readable Git output and path-safe parsing. Avoid shell-string interpolation for file paths.

Request checklist

• I've searched existing Issues and Discussions for duplicates
• This describes a specific problem with clear context and impact

Zoo Code Task Links (optional)

No response

Optional (for contributors): You can stop here if you're just proposing the improvement.

Acceptance criteria (optional)

• Staged changes are used when present.
• Unstaged and untracked changes are used only when no staged changes exist.
• Added, modified, deleted, renamed, copied, binary, and untracked files are represented safely.
• Zoo ignore filters are not applied to this Git-source-of-truth workflow.
• Unit tests cover parsing and staged/unstaged precedence.

Proposed approach (optional)

Add a Git context collector independent of VS Code UI and AI generation. Prefer Git porcelain/name-status formats and test the parser directly.

Trade-offs / risks (optional)

Path parsing is the main risk, especially renamed/copied paths and paths containing whitespace or unusual characters.