Add update_task fields, list_tasks, and update_todo tools

- update_note: extend with status/priority/due_date fields so task attributes
  can be changed via chat (mark done, set priority, move due date). body is now
  optional — task field updates work without touching content.
- list_tasks: new core tool with status/priority/due_before/due_after/limit
  filters backed by list_notes(is_task=True). Enables queries like
  "overdue tasks", "high priority tasks", "what's in progress".
- update_todo: new CalDAV tool to modify VTODO summary, due date, description,
  and priority — follows update_event pattern (modify component, rebuild ical,
  save). Completes the CalDAV todo CRUD suite.
- tools.py: add update_todo import + execute case (type: todo_updated)
- llm.py: add list_tasks and update_todo to available actions + guidance
- intent.py: routing rules for mark-done/priority/due-date → update_note,
  overdue/in-progress/high-priority queries → list_tasks, CalDAV todo updates
  → update_todo
- ToolCallCard.vue: tasks list block (linked titles + due + priority badges),
  todo_updated label, tool-task-priority CSS classes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/services/llm.py

@@ -250,21 +250,22 @@ async def build_context(
     tool_lines = [
         "You have access to tool functions. You MUST use them when the user asks you to create, add, find, schedule, or search for anything.",
         "CRITICAL: Call the tool functions directly. NEVER write out function calls as text or code. NEVER describe what you would do — just do it.",
-        "Available actions: create_task, create_note, update_note, search_notes.",
+        "Available actions: create_task, create_note, update_note, list_tasks, search_notes.",
     ]
     if has_caldav:
         tool_lines[-1] = (
-            "Available actions: create_task, create_note, update_note, search_notes, "
+            "Available actions: create_task, create_note, update_note, list_tasks, search_notes, "
             "create_event, list_events, search_events, update_event, delete_event, "
-            "list_calendars, create_todo, list_todos, complete_todo, delete_todo."
+            "list_calendars, create_todo, list_todos, update_todo, complete_todo, delete_todo."
         )
         tool_lines.append("For calendar events, use ISO 8601 datetime format (e.g. 2026-09-30T00:00:00).")
         tool_lines.append("CalDAV todos are separate from app tasks. Use create_task for app tasks, create_todo for CalDAV todos.")
         tool_lines.append("When the user says 'remind me' with a time before an event, use the reminder_minutes parameter.")
     tool_lines.append("For relative dates like 'Friday' or 'next week', resolve them to YYYY-MM-DD format.")
     tool_lines.append(
-        "Use update_note to edit/expand an existing note. "
+        "Use update_note to edit/expand an existing note OR to update a task's status/priority/due_date. "
         "Use create_note ONLY for genuinely new notes with a different title. "
+        "Use list_tasks to find tasks by status, priority, or due date (e.g. overdue, high priority, in progress). "
         "If a note was created earlier in the conversation and the user provides more content for it, use update_note."
     )
     tool_guidance = "\n".join(tool_lines)