Add read-only git_blame and git_show tools

[ayobamiseun]

What

Adds git_blame and git_show to the read-only git toolkit in coworker/tools/git.py, whose module docstring already frames the purpose ("see how a file came to be the way it is before changing it") — blame and show complete the natural loop: git_log (which commits) → git_blame (who last touched this line, in which commit) → git_show (what that commit actually changed).

• git_blame(path, start_line?, end_line?) → per-line {line, hash, author, date, content}, parsed from --line-porcelain. Whole-file blame is capped at 300 lines with a note pointing at the range parameters.
• git_show(ref=HEAD, path?) → the commit's message, diffstat, and patch, capped at 8000 chars with a note pointing at path scoping.

Both follow the git_log template exactly: schema dict, ToolMetadata(category="git", risk_level="low", requires_approval=False), __coworker_schema__. The subprocess runner is factored out and shared; git_log's behavior and error strings are unchanged. Commit/push stay excluded per the module's existing contract — they route through run_shell and its approval gate.

Security note: a ref is model input landing on a subprocess argv. git_show rejects anything that doesn't look like a ref — a leading - would be parsed as a git option (--output writes arbitrary files), and whitespace/metachars are refused. Paths are always passed after --, as git_log already does. There's a test asserting the injection refusal.

Wiring (no new surface)

The git capability builder already returns everything from git_tools(), so the new tools reach exactly the personas/agents that had git before (Code, and the read-only Explore subagent) — no catalog, persona, or permission changes beyond:

• tests/test_catalog.py: the Phase-0 exact-toolset equivalence set extended with the two names.
• The git capability description now says "(status, diff, log, blame, show)".

Tests

Six new tests in tests/test_code_tools.py following the existing git_log fixtures: two-author repo → blame attributes each line to the right author/commit; range scoping; error paths (no repo, missing file, empty path); show returns subject + patch + diffstat; relative refs (HEAD~1) with path scoping; and option-injection refusal.

Verification

$ .venv/bin/python -m pytest tests/test_code_tools.py -q
20 passed

$ .venv/bin/python -m pytest tests -q
895 passed, 1 skipped

No UI change (tools surface through the existing tool-approval cards), so no screenshots.