-- Anchor a persona conversation on a point in an SMS thread.
--
-- "Continue with persona" starts a persona chat from a long-pressed message
-- in the Messages screen. The conversation remembers which thread and which
-- message it grew out of, so the SMS screen can list the persona chats that
-- belong to a thread and the chat screen can show where it picked up.
--
-- `anchor_context` is a snapshot of the thread leading up to the anchor,
-- rendered once at creation and spliced into the system prompt on every
-- turn. Snapshotting (rather than refetching per turn) keeps the prompt
-- 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;
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);