feat: real git introspection (changes, wip, file-diff, history ops)

[fdatoo]

Summary

• fbi/git.gleam — shell-out wrapper (git -C repo_path) with typed GitError
• fbi/git/parse.gleam — pure text parsers: log porcelain, name-status, numstat, unified diff hunks, status porcelain v2
• fbi/git/repo.gleam — domain ops against the bare repo: commits_on_branch, commit_files, file_diff, branch_base_ahead_behind, wip_files / wip_file_diff / wip_patch (reading refs/fbi/wip-snapshot)
• fbi/git/mutex.gleam — per-run OTP actor serialising concurrent history ops
• fbi/git/history_ops.gleam — squash_local, mirror_rebase, sync_in_container, merge_in_container, dispatch_polish, dispatch_merge_conflict
• fbi/handlers/history.gleam — real POST /api/runs/:id/history dispatching all five ops
• handlers/changes.gleam — real /changes, /commits/:sha/files, /file-diff
• handlers/wip.gleam — real /wip, /wip/file, /wip/patch, /wip/discard (via docker exec)
• docker.gleam — exec_container (create → start → inspect exec lifecycle)
• db/runs.gleam — children_of, insert_polish_run, insert_merge_conflict_run, count_active_children
• context.gleam / fbi.gleam — history_mutex field; actor started at boot
• priv/static/wip-snapshotter.sh — in-container daemon writing refs/fbi/wip-snapshot to safeguard every 5 s
• supervisor.sh — starts snapshotter; handles polish / merge-conflict kinds
• run/worker.gleam — binds snapshotter + polish-prompt into containers; exports FBI_KIND

All stub handlers replaced. 67 tests pass (2 pre-existing NIF failures unrelated to this change).

Test plan

• gleam test — 67 passed, 2 pre-existing failures only
• GET /api/runs/:id/changes returns branch_name, commits[], branch_base, uncommitted[], children[]
• GET /api/runs/:id/commits/:sha/files returns real file list with status + line counts
• GET /api/runs/:id/file-diff?path=...&ref=<sha> returns hunk list
• GET /api/runs/:id/wip returns ok: true with files once snapshotter has fired (≥5 s after container start)
• POST /api/runs/:id/wip/discard clears working-tree changes via docker exec
• POST /api/runs/:id/history with {"op":"polish"} spawns a child run and returns {"kind":"agent","child_run_id":N}

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