feat(persona-chat): practice mode — play the contact

An anchored conversation now has a mode. 'about' is the existing behaviour (a
persona talks with the user about the thread). 'as_contact' has the model play
the contact so the user can rehearse the conversation: its own prompt (stay in
the contact's voice, don't invent facts, coach on 'out of character'), no
persona prompt, and only the thread tools — no memory, library, calendar or
other-thread access, and no knowledge writes.

The shared persona/file-chat loop now refuses tool calls that weren't in the
turn's catalog, so the gates hold even when a model names a tool it wasn't
offered.

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

migrations/2026-10-19-000000_persona_chat_sms_anchor/down.sql

@@ -1,4 +1,5 @@
 DROP INDEX IF EXISTS idx_persona_chat_anchor_contact;
+ALTER TABLE persona_chat_conversations DROP COLUMN anchor_mode;
 ALTER TABLE persona_chat_conversations DROP COLUMN anchor_context;
 ALTER TABLE persona_chat_conversations DROP COLUMN anchor_ts;
 ALTER TABLE persona_chat_conversations DROP COLUMN anchor_message_id;

migrations/2026-10-19-000000_persona_chat_sms_anchor/up.sql

@@ -11,6 +11,10 @@
 -- prefix byte-stable for KV-cache reuse and keeps the persona's view of the
 -- thread fixed even if SMS-API is re-imported underneath it.
 --
+-- `anchor_mode` is how the chat relates to the thread: 'about' (a persona
+-- talks with the user about it) or 'as_contact' (the model plays the
+-- contact so the user can practise the conversation). NULL reads as 'about'.
+--
 -- All nullable: an unanchored (open) persona chat leaves them NULL.
 
 ALTER TABLE persona_chat_conversations ADD COLUMN anchor_contact_id BIGINT;
@@ -18,6 +22,7 @@ ALTER TABLE persona_chat_conversations ADD COLUMN anchor_contact_name TEXT;
 ALTER TABLE persona_chat_conversations ADD COLUMN anchor_message_id BIGINT;
 ALTER TABLE persona_chat_conversations ADD COLUMN anchor_ts BIGINT;
 ALTER TABLE persona_chat_conversations ADD COLUMN anchor_context TEXT;
+ALTER TABLE persona_chat_conversations ADD COLUMN anchor_mode TEXT;
 
 CREATE INDEX idx_persona_chat_anchor_contact
     ON persona_chat_conversations (user_id, anchor_contact_id, updated_at DESC);