[zheng-weihao]

Problem

An exact fetch refspec pointing at a branch the remote no longer has makes every bare git fetch fail with fatal: couldn't find remote ref — in every worktree of the pool. Two sources produced such entries:

1. Residue: orbit_add_fetch_refspec entries outlive their remote branch (typical: auto-deleted on PR merge). Nothing ever removed them.
2. Pre-registration (found by dogfooding this fix): switch -c registered a refspec for the not-yet-pushed branch, breaking every bare fetch between switch and first push — the fix branch itself broke the pool's fetch while this PR was being written.

Fix

Every orbit command that already fetches opportunistically — orbit sync, orbit info, the orbit context --startup reignite block — plus orbit prune now reconciles fetch refspecs with the remote first (one ls-remote per repo), then fetches bare (git fetch origin):

• Remove entries whose remote branch is gone, along with their stale refs/remotes/origin/<branch> ref. → removed stale fetch refspec: <branch>
• Register entries for locally-tracked branches that now exist remotely (scoped/raw branch pushed since last run); the bare fetch materializes the tracking ref immediately. → added fetch refspec: <branch>

Changes print on stderr (report commands) so stdout/JSON shapes are untouched; prune --dry-run reports would remove/add ....

Push-time UX preserved without the broken window: switch -c no longer pre-registers (the branch doesn't exist remotely yet — the refspec is the bug), but it still wires the upstream config up front, so push/pull work immediately; the tracking ref materializes at the first fetching command after the push — session start, orbit info, or orbit sync — or instantly via git fetch origin <branch>. (A git pre-push hook could materialize at push time, but orbit does not install git hooks into pool repos — it would clash with repos' own hook setups.)

Supporting details:

• Removal rebuilds the value list for exact match — git config --unset treats the refspec's leading + as an invalid regex, and --fixed-value needs git ≥ 2.26 (orbit supports ≥ 2.20).
• Reconciliation is skipped entirely when the remote is unreachable, so unverifiable refspecs are never removed by mistake.
• Bare orbit context (cruise block) and orbit status stay purely local (no fetch, no reconcile).

Verified end-to-end on a live pool: planted residue → bare fetch fails → orbit sync removes it → bare fetch works; and the pre-registration residue left by this branch's own switch -c was cleaned the same way.

Tests

• 09_prune: removes remote-deleted residue (asserts bare fetch broken before, healed after), --dry-run reports without mutating, removes legacy pre-registered entries while keeping the local branch
• 18_sync: same removal via sync, keeps live refspecs
• 06_worktree_integration: switch -c does not pre-register; push → orbit sync registers the refspec and materializes origin/<branch>
• 12_context: push → session-start reignite registers + materializes (the agent-facing path)

Full suite: 215/215 ok, make lint clean.