fix(ui): surface precondition hints for silently-disabled review buttons (UX-Audit #2)

Cancel now shows an amber hint + tooltip while a merge drain is in
progress (ShowMergeDrainHint mirrors IsMergeDraining). Continue,
Reset & Retry, Open Diff, Worktree, and Review Combined Diff gain
ToolTip.Tip + ToolTip.ShowOnDisabled explaining why they're locked.
No CanExecute or behavior changed, only added communication.

src/ClaudeDo.Localization/locales/en.json

@@ -253,7 +253,9 @@
     }
   },
   "agent": {
-    "openWorktreeTip": "Open worktree in file explorer"
+    "openWorktreeTip": "Open worktree in file explorer — requires an active worktree",
+    "openDiffTip": "Open the diff — requires a worktree or a merged commit range",
+    "reviewCombinedDiffTip": "Only available for planning tasks with subtasks or child outcomes"
   },
   "notes": {
     "today": "Today",
@@ -268,7 +270,10 @@
     "chipFailed": "FAILED",
     "reviewContinueTip": "Send this feedback and re-run the task",
     "reviewResetTip": "Discard all changes and reset the task to Idle",
-    "reviewFeedbackPlaceholder": "Feedback for next run"
+    "reviewFeedbackPlaceholder": "Feedback for next run",
+    "mergeDrainHint": "Merge is draining subtasks — Cancel unlocks once it finishes",
+    "continueNoSessionTip": "Resumes the last run — needs a previous session to continue",
+    "resetAndRetryInteractiveTip": "Interactive session is open — close it first"
   },
   "missionControl": {
     "windowTitle": "Mission Control",