fix(watcher): stop cross-worktree fan-out via shared commondir

[pauldambra]

Problem

Starting a new worktree task gets slower as the number of existing worktrees grows. Reported from a fresh machine (few worktrees) feeling dramatically faster than an older one with many worktrees.

Root cause: linked worktrees share the main repo's .git as their commondir, and each worktree's watchRepo recursively watches that commondir. So mutating any worktree's admin dir under .git/worktrees/ — notably creating a new worktree (which writes .git/worktrees/<name>/HEAD) — woke every other worktree's watcher. Each wake fired a server-side branch re-check (a git rev-parse subprocess) and a renderer branch-query invalidation. With N worktrees, one git operation triggered ~N subprocesses + N invalidations, so the per-event cost grew linearly with worktree count — and creating a task is exactly the moment that writes to the shared .git.

Changes

Exclude the worktrees/ admin subtree (**/worktrees/**) from the git-dir watches in WatcherService.watchRepo.

• A worktree's own admin dir is still watched directly as its gitDir (rooted inside worktrees/<name>, where the pattern matches nothing), so its own HEAD changes are still observed.
• Shared refs (refs/heads, packed-refs) live outside worktrees/, so commits/branch changes still propagate to sibling worktrees as before.
• Only cross-worktree admin noise is dropped.

Note: the ignore-option mechanism is load-bearing here — it's relative to each watch root, so it suppresses the subtree only for the shared commondir watch and not for a worktree's own gitDir watch. Filtering by an absolute-path substring instead would wrongly drop a worktree's own HEAD events.

How did you test this?

• Added service.test.ts covering the ignore wiring for both a linked worktree (commondir + own gitDir get the worktrees ignore; working tree keeps node_modules/.git ignores) and a non-worktree repo. Both pass via vitest run src/services/watcher/service.test.ts.
• biome lint clean on the watcher dir; tsc clean for the changed files (remaining repo typecheck errors are unbuilt sibling-package dist/, unrelated to this change).

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog from a Slack thread

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 99d172d.}

[greptile-apps]

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