#1038 Add the read-only git adapter for worktree and base-branch ground truth

[williamthorsen]

What

Fleet's lane view now reflects each worktree's real git state (the checked-out branch, the number of uncommitted files, and how far ahead or behind the base branch it sits), refreshed on a configurable interval. A lane whose worktree has been deleted now closes on its own, instead of lingering indefinitely.

Why

Fleet's lane state derived entirely from emitted lifecycle events, leaving the working tree itself invisible: uncommitted changes and base-branch advance had no ground truth. And a session that ended abnormally, with no session.ended hook, kept its lane open indefinitely even after its worktree was gone.

Details

🎉 Features

• A read-only git adapter polls each lane's worktree and overlays a git block on the lanes snapshot and SSE stream: checked-out branch, working-tree change count, ahead/behind against the base branch, and the base ref compared against, null until the lane has been probed. It never fetches or mutates a repository; base-branch advance is read from the local remote-tracking ref that worktrees share.
• A lane whose worktree no longer exists closes as worktree-gone, the backstop for a session that ended without a session.ended event.
• An unreadable or non-repository worktree degrades only that lane's git fields, never the rest of the fleet.
• The lane fold retains each session's working directory, and resolveLaneCwd exposes the lane's worktree path for the adapter to probe.
• FLEET_GIT_POLL_MS (default 15000 ms) sets the poll interval.

♻️ Refactoring

• The composite lane key ({repo}/{branch}) that indexes lanes across the store, the adapter's targets, and the snapshot lookup now resolves through one buildLaneKey helper, so the format cannot drift between its producer and consumers.

🧪 Tests

• Behavioral coverage of probe outcomes (clean, dirty, ahead/behind, detached HEAD, no remote base, missing worktree, non-repository directory), adapter-loop behavior (first-pass exposure, target eviction, degraded probe, non-overlapping passes), the snapshot overlay, config resolution, the lane-fold cwd retention, and an end-to-end server path that surfaces git data and then closes a deleted worktree.

Closes #1038

[github-actions]

Dependency audit

Production dependency audit passed.