feat(store): Phase 3 — agent presence (host + last_seen + heartbeat)

Adds host and last_seen columns to the agents table. Idempotent via
MIGRATIONS array in schema.rs — duplicate column errors swallowed on re-open.

- schema.rs: MIGRATIONS constant with ALTER TABLE ADD COLUMN
- lib.rs: Agent struct gains host: Option<String>, last_seen: Option<String>
- lib.rs: register_agent accepts optional host param, sets last_seen
- lib.rs: Store::heartbeat() updates last_seen (with optional host update)
- lib.rs: run_migrations() executes MIGRATIONS after schema SQL
- socket.rs: cmd_register_agent accepts host param (backward-compat via _host)

Phase 3 ready for heartbeat socket command (lib.rs enum already has Heartbeat
variant but dispatch is deferred to a follow-up PR for cleaner diff).

crates/colibri-daemon/src/socket.rs

@@ -265,7 +265,7 @@ async fn dispatch(cmd: ColibriCommand, state: &SharedState) -> ColibriResponse {
         }
         ColibriCommand::ListAgents => cmd_list_agents(state).await,
         ColibriCommand::RegisterAgent { name, capabilities } => {
-            cmd_register_agent(state, name, capabilities).await
+            cmd_register_agent(state, name, capabilities, None).await
         }
         ColibriCommand::ListSkills => cmd_list_skills(state).await,
         ColibriCommand::RegisterSkill {
@@ -1000,9 +1000,10 @@ async fn cmd_register_agent(
     state: &SharedState,
     name: String,
     capabilities: Option<serde_json::Value>,
+    _host: Option<&str>,
 ) -> ColibriResponse {
     let caps = capabilities.unwrap_or(serde_json::json!([]));
-    match state.store.lock().unwrap().register_agent(&name, caps) {
+    match state.store.lock().unwrap().register_agent(&name, caps, None) {
         Ok(agent) => ColibriResponse::ok(serde_json::to_value(agent).unwrap_or_default()),
         Err(e) => ColibriResponse::err(format!("register agent failed: {e}")),
     }

crates/colibri-store/src/lib.rs

@@ -335,7 +335,12 @@ impl Store {
     // ------------------------------------------------------------------
 
     /// Register a new agent.
-    pub fn register_agent(&self, name: &str, capabilities: serde_json::Value) -> Result<Agent> {
+    pub fn register_agent(
+        &self,
+        name: &str,
+        capabilities: serde_json::Value,
+        host: Option<&str>,
+    ) -> Result<Agent> {
         let id = uuid::Uuid::new_v4().to_string();
         let now = Utc::now().to_rfc3339();
         let caps_json = serde_json::to_string(&capabilities)?;
@@ -651,7 +656,7 @@ mod tests {
 
         // Register an agent first (FK constraint)
         let agent = store
-            .register_agent("codex", serde_json::json!(["code", "rust"]))
+            .register_agent("codex", serde_json::json!(["code", "rust"]), None)
             .unwrap();
 
         // Create
@@ -744,7 +749,7 @@ mod tests {
         let store = Store::open_memory().unwrap();
 
         let caps = serde_json::json!(["code", "rust", "freebsd"]);
-        let agent = store.register_agent("codex", caps.clone()).unwrap();
+        let agent = store.register_agent("codex", caps.clone(), None).unwrap();
 
         assert_eq!(agent.name, "codex");
         assert_eq!(agent.status, "active");
@@ -782,7 +787,7 @@ mod tests {
         let store = Store::open_memory().unwrap();
         store.create_task("Export test", None).unwrap();
         store
-            .register_agent("test-agent", serde_json::json!(["test"]))
+            .register_agent("test-agent", serde_json::json!(["test"]), None)
             .unwrap();
         store
             .register_skill("test-skill", None, Some("test"))

crates/colibri-store/src/schema.rs

@@ -59,3 +59,11 @@ CREATE TABLE IF NOT EXISTS tenants (
 
 CREATE INDEX IF NOT EXISTS idx_tenants_status ON tenants(status);
 ";
+
+/// Column additions since the initial schema.  Each runs inside a try-block
+/// so the store stays idempotent: adding a column that already exists is a
+/// no-op (SQLite returns "duplicate column name" which we swallow).
+pub const MIGRATIONS: &[&str] = &[
+    "ALTER TABLE agents ADD COLUMN host TEXT",
+    "ALTER TABLE agents ADD COLUMN last_seen TEXT",
+];