git: coalesce remote fetches per repository and window

[wheregmis]

Summary

Modeled on pingdotgg/t3code#14673, which cut background GitHub polling ~74% by coalescing calls: every refresh path that syncs remote refs ran its own git fetch --prune, and bursts turned into sustained remote traffic. The triggers — Inspect { sync_remote } from project switches and the four panel-open actions, WorkspaceChanged { git_dirty } reloads (a fetch's own ref writes feed the watcher back as git_dirty), and per-worktree GitOperation::Fetch calls — all funnel through threadlane_git::sync_remote/fetch, so the fix lives there and covers daemon and local mode alike.

• RemoteFetchSlot per repository, keyed by git rev-parse --git-common-dir: linked worktrees share refs/remotes/*, so sessions in sibling worktrees of one repo now share a single fetch pipeline.
• run_remote_fetch gives each slot two behaviors: callers arriving while a fetch is in_flight wait and share its result (success or error) instead of racing a second process on .git locks; and a completed Ok younger than REMOTE_SYNC_TTL (60s) satisfies later callers outright.
• sync_remote uses the 60s window — background refreshes collapse to ≤1 remote call per repo per minute.
• fetch (the panel's Fetch action, worktree_bases) passes ttl = ZERO: explicit requests still contact the remote every time, but coalesce with an in-flight run and re-arm the window for the background path.
• pull calls note_remote_fetched on success — a pull already fetched, so the next background sync skips its own.
• Failed fetches are returned to queued callers but never freshness-stamped, so the next sync retries immediately.

Testing

cargo nextest run -p threadlane-git — 88 tests pass, including new coverage: TTL skip, explicit-fetch bypass/re-arm, cross-worktree slot sharing, retry-after-failure, and queued waiters sharing one in-flight run (REMOTE_FETCH_RUNS counts real fetch spawns). cargo check -p threadlane-gpui and git diff --check clean.

Link to Devin session: https://app.devin.ai/sessions/922c096109634db8a2b783534aa62196
Open in Devin Desktop: https://app.devin.ai/desktop/session/922c096109634db8a2b783534aa62196?variant=devin
Requested by: @wheregmis

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Review	Status	Commit	Review trigger
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• crates/threadlane-git/src/git.rs
• crates/threadlane-git/src/tests.rs

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