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>

src/ai/persona_chat.rs

@@ -242,6 +242,9 @@ pub struct PersonaChatAnchorView {
     pub timestamp: i64,
     /// The anchor message as "Speaker: text", recovered from the snapshot.
     pub preview: String,
+    /// `about` (a persona discusses the thread) or `as_contact` (the model
+    /// plays the contact for practice).
+    pub mode: crate::database::AnchorMode,
 }
 
 impl From<&crate::database::SmsAnchor> for PersonaChatAnchorView {
@@ -252,6 +255,7 @@ impl From<&crate::database::SmsAnchor> for PersonaChatAnchorView {
             message_id: a.message_id,
             timestamp: a.timestamp,
             preview: crate::ai::sms_anchor::anchor_preview(&a.context),
+            mode: a.mode,
         }
     }
 }
@@ -596,6 +600,7 @@ impl PersonaChatSession {
         persona_id: &str,
         contact_id: i64,
         message_id: i64,
+        mode: crate::database::AnchorMode,
     ) -> Result<String, PersonaChatError> {
         let persona_id = persona_id.trim();
         {
@@ -630,6 +635,7 @@ impl PersonaChatSession {
             message_id,
             timestamp: window.messages[window.anchor_index].timestamp,
             context,
+            mode,
         };
 
         let mut dao = self.persona_dao.lock().expect("persona_dao poisoned");
@@ -935,6 +941,12 @@ impl PersonaChatSession {
         );
         gate_opts.sms_thread_tools = row.anchor.is_some();
         let tools = InsightGenerator::build_tool_definitions(gate_opts);
+        // Practice mode narrows the catalog to the thread itself — no
+        // memory, library or calendar access for a simulated contact.
+        let tools = match &row.anchor {
+            Some(anchor) => crate::ai::sms_anchor::tools_for_mode(tools, anchor.mode),
+            None => tools,
+        };
 
         // Append the new user turn + apply the per-turn system-prompt
         // override. Per-persona overrides are NOT persisted (the persona
@@ -1368,6 +1380,9 @@ pub struct PersonaChatCreateRequest {
 pub struct PersonaChatAnchorRequest {
     pub contact_id: i64,
     pub message_id: i64,
+    /// `about` (default) or `as_contact`.
+    #[serde(default)]
+    pub mode: crate::database::AnchorMode,
 }
 
 /// Query params for GET /persona_chat/conversations.
@@ -1489,6 +1504,7 @@ pub async fn persona_chat_create_conversation_handler(
                     &request.persona_id,
                     anchor.contact_id,
                     anchor.message_id,
+                    anchor.mode,
                 )
                 .await
         }
@@ -2018,6 +2034,24 @@ mod tests {
         assert!(tracker.claim("conv-a", "t7").is_ok());
     }
 
+    #[test]
+    fn create_request_anchor_mode_defaults_to_about() {
+        use crate::database::AnchorMode;
+        let req: PersonaChatCreateRequest = serde_json::from_str(
+            r#"{"persona_id":"journal","anchor":{"contact_id":42,"message_id":812}}"#,
+        )
+        .unwrap();
+        assert_eq!(req.anchor.unwrap().mode, AnchorMode::About);
+        let req: PersonaChatCreateRequest = serde_json::from_str(
+            r#"{"persona_id":"journal","anchor":{"contact_id":42,"message_id":812,"mode":"as_contact"}}"#,
+        )
+        .unwrap();
+        assert_eq!(req.anchor.unwrap().mode, AnchorMode::AsContact);
+        let req: PersonaChatCreateRequest =
+            serde_json::from_str(r#"{"persona_id":"journal"}"#).unwrap();
+        assert!(req.anchor.is_none());
+    }
+
     #[test]
     fn persona_chat_turn_request_deserializes_snake_case_wire_body() {
         // Pins the mobile-client contract: snake_case keys, same as the
@@ -2273,12 +2307,14 @@ mod tests {
                 message_id: 812,
                 timestamp: 1_700_000_000,
                 preview: "Sam: see you at 6".to_string(),
+                mode: crate::database::AnchorMode::AsContact,
             }),
         };
         let json = serde_json::to_value(&summary).unwrap();
         assert_eq!(json["anchor"]["contact_id"], 42);
         assert_eq!(json["anchor"]["message_id"], 812);
         assert_eq!(json["anchor"]["preview"], "Sam: see you at 6");
+        assert_eq!(json["anchor"]["mode"], "as_contact");
         assert_eq!(json["conversation_id"], "conv-1");
         assert_eq!(json["title"], "June recap");
         assert_eq!(json["persona_id"], "journal");