aa-bridge: set committer identity to aa-bridge on spawned claude
Author stays as whatever the cwd's git config says (@jeffrey), but
the committer is now aa-bridge <aa@aesthetic.computer> on every
commit made through the bridge. Standard "authored-by-X,
committed-by-Y" pattern — trivially filterable via
`git log --committer=aa-bridge`, visible in `--pretty=fuller` and
in the GitHub UI. Overridable via AA_GIT_COMMITTER_NAME / _EMAIL.
Also exports AA_BRIDGE=1 so downstream tools (hooks, scripts) can
detect the context.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>