Phase 7: Migrate PR mutations to useMutation hooks

[jennings]

Migrate PR mutations from methods on PRReviewStore to useMutation hooks. Delete the mutation state (merging, mergeError, dequeueing, etc.) from the store.

Methods migrated

Each becomes a useMutation hook in src/browser/lib/mutations.ts:

• mergePR → useMergePR
• dequeuePR → useDequeuePR
• closePR → useClosePR
• reopenPR → useReopenPR
• deleteBranch → useDeleteBranch
• restoreBranch → useRestoreBranch
• convertToDraft → useConvertToDraft
• markReadyForReview → useMarkReadyForReview
• updatePR → useUpdatePR
• updateBranch → useUpdateBranch
• approveWorkflows → useApproveWorkflows
• getRepoBranches → useRepoBranches (query, not mutation, but currently lives with these)

Store fields removed

• merging, mergeError, mergeMethod (setter stays — setMergeMethod — since mergeMethod is a UI selection, not fetched state — reconsider during implementation)
• dequeueing, closingPR, reopeningPR, deletingBranch, restoringBranch
• convertingToDraft, convertingToDraftError
• markingReady, markingReadyError
• approvingWorkflows, updatingPR

Consumers read mutation.isPending / mutation.error instead.

Optimistic + invalidation pattern

Each mutation's onMutate should apply the same optimistic setQueryData calls the store used to do inline. onSuccess (or onSettled) invalidates:

• mergePR → invalidate PR, timeline, comments, reviews, checks.
• closePR / reopenPR → invalidate PR + timeline.
• deleteBranch / restoreBranch → invalidate branch status + PR.
• convertToDraft / markReadyForReview → invalidate PR + timeline.
• updatePR → invalidate PR + timeline.
• approveWorkflows → invalidate workflow runs + checks.

Consumer changes

• pr-overview.tsx, pr-edit-dialog.tsx, and anywhere a merge/close/reopen button lives — swap method calls for mutation .mutate(); swap in-flight flag reads for mutation.isPending.

Tests

Per mutation hook:

• Optimistic-update path: cache reflects the change immediately.
• Success path: cache reconciled with server response, correct queries invalidated.
• Error path: optimistic update rolled back, error surfaces to mutation.error.

Delete corresponding store mutation-method tests.

Part of #351

Depends on #353.