fix(llm): correct context sizing, honor think requests, broaden delete

Three related fixes uncovered while benchmarking qwen3:14b against 8b:

- pick_num_ctx was only counting message content, missing the ~15K
  tokens of tool schemas. num_ctx=8192 was being selected while actual
  prompt_tokens hit 14K+, causing silent prompt truncation on every
  tool-using request. Now includes json.dumps(tools) in the estimate.
  KV cache priming in app.py and routes/settings.py also fetches tools
  so the primed num_ctx matches what real chat requests will use.

- _should_think's heuristic classifier was overriding explicit
  think=true requests from the frontend toggle and MCP, gating on
  message length and regex patterns. Now a pass-through — the caller
  is the source of truth. quick_capture hardcodes think=False since
  it's a fast classification path that was relying on the old gating.

- delete_note description only mentioned "note or task", so the model
  refused to call it for entries created by save_person / save_place /
  create_list. Description now explicitly lists all five note_types it
  handles.

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

src/fabledassistant/services/tools/notes.py

@@ -391,9 +391,9 @@ async def list_notes_tool(*, user_id, arguments, **_ctx):
 
 @tool(
     name="delete_note",
-    description="Delete a note or task permanently. Use ONLY when the user explicitly asks to delete or remove an item. Always confirm with the user first — this cannot be undone.",
+    description="Delete any item from the user's knowledge base permanently — notes, tasks, persons (created via save_person), places (created via save_place), and lists (created via create_list) are all stored as notes and use this single delete tool. Use ONLY when the user explicitly asks to delete or remove an item. Always confirm with the user first — this cannot be undone.",
     parameters={
-        "query": {"type": "string", "description": "Title or keyword to find the note or task to delete"},
+        "query": {"type": "string", "description": "Title or keyword to find the item to delete (works for notes, tasks, persons, places, and lists)"},
         "confirmed": {"type": "boolean", "description": "Must be true — only set after the user has explicitly confirmed they want this item deleted."},
     },
     required=["query"],