fix: scope react-query cache invalidation per mutation

[uditdc]

Closes #12

queries.ts's useGitMutation called onSettled: () => qc.invalidateQueries() unconditionally, invalidating every cache entry (log, refs, repo, status, worktrees, stashes, diffs) on every mutation — so staging a single file refetched the branch list, commit log, and stashes along with it.

Each mutation now declares a ChangeScope ('refs' | 'index' | 'worktree' | 'all') mirroring the scope its own server route already emits over SSE (bus.emit(...) in src/server/routes/*.ts), and a shared invalidateScope helper invalidates only the query groups that scope covers:

• checkout, createBranch, createTag, fetchRemote, push → refs (log/refs/repo)
• stage, unstage → index (status/worktrees/diff-working)
• commit, uncommit, pull → all (both groups)
• stashPush, stashApply, stashPop → all + stashes; stashDrop → refs + stashes (stashes list is only invalidated by the four stash mutations)

Acceptance criteria

• Staging a file no longer refetches unrelated queries (e.g. branch list) — stage/unstage now only invalidate the index scope group (status, worktrees, diff/working); verified with a unit test (tests/query-invalidation.test.ts) asserting the exact key set invalidated per scope, and by reading through every mutation's mapping against its server route's bus.emit call.
• No regression in data freshness after commit/push/checkout/etc. — each mutation's scope was derived directly from the same scope its server route already emits for the SSE-driven cross-tab refresh (src/server/routes/actions.ts, remote.ts, stash.ts), so mutation-triggered invalidation matches exactly what the server itself considers affected by that action.

pnpm typecheck, pnpm test, and pnpm build all pass.

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