fix(persona-chat): review fixes before merge

- Thread tools are bound server-side to the chat's own thread. The model
  used to pass contact_id, so practice mode could read any conversation it
  named; a model-supplied contact_id is now only checked, never trusted, and
  the prompts no longer hand it one.
- Practice chats need no persona. They're stored under PRACTICE_PERSONA_ID
  and run with an inert stand-in, so deleting or editing personas can't hide
  or break them. persona_id is optional on create for as_contact.
- The per-thread list filters in SQL (list_persona_chats_for_contact, served
  by idx_persona_chat_anchor_contact) instead of decoding every transcript.
- fetch_mms_image checks the content type and a 20 MB cap before reading
  the body, so a video attachment isn't downloaded in full.
- get_photos_near_time unions tags over every path sharing the content, per
  the multi-library model.
- look_at_photo only opens indexed photos, so an excluded folder can't be
  read by guessing a path.
- One date parser: get_photos_near_time uses date_info::parse_center.
- Practice prompt source rewrapped.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -578,13 +578,19 @@ GET  /insights/chat/history?path=... (rendered transcript with tool invocations)
 POST /insights/chat/rewind           (truncate transcript at a rendered index)
 
 // Persona chat — "Continue with persona" from an SMS thread (src/ai/sms_anchor.rs)
-POST /persona_chat/conversations     (body: { persona_id, anchor?: { contact_id, message_id } };
-                                      anchor snapshots ~40 SMS-API messages up to message_id into
-                                      the row, 502 if SMS-API can't read the thread)
-GET  /persona_chat/conversations?contact_id=N   (only chats anchored on that thread)
-//   Anchored turns append the thread + chat-register guidance to the persona prompt and add
-//   the get_thread_messages / look_at_message_image / get_photos_near_time / look_at_photo
-//   tools (ToolGateOpts.sms_thread_tools). Image tools use the LOCAL vision model.
+POST /persona_chat/conversations     (body: { persona_id, anchor?: { contact_id, message_id,
+                                      mode?: "about" | "as_contact" } }; anchor snapshots ~40
+                                      SMS-API messages up to message_id into the row, 502 if
+                                      SMS-API can't read the thread. `as_contact` (practice) needs
+                                      no persona: stored under PRACTICE_PERSONA_ID, listed as
+                                      "Practice")
+GET  /persona_chat/conversations?contact_id=N   (only chats anchored on that thread; DAO query)
+//   Anchored turns append the thread + mode guidance to the prompt and add the thread
+//   tools (ToolGateOpts.sms_thread_tools): get_thread_messages / look_at_message_image are
+//   bound server-side to the chat's own thread (a model-named contact_id is refused), and
+//   about-mode also gets get_photos_near_time / look_at_photo. Practice keeps only the two
+//   thread tools. Image tools use the LOCAL vision model; MMS downloads are image-only and
+//   capped (MMS_IMAGE_MAX_BYTES); look_at_photo only opens indexed photos.
 
 // User AI Prefs (owner defaults + passive mirror; src/prefs.rs)
 GET  /prefs                          (current row; auth)