add token infrastructure: service_users table, localpart helper

new SQLite table service_users maps OIDC identities (matrix localpart)
to service-specific usernames, handling auth boundary mismatches.
localpart() extracts the username from a matrix user ID.

delete_all_conversations() added for bulk reset after agent recreation.
all delete_* methods now log failures instead of silently discarding.
removed dead user_tokens table (tokens now live in vault).

src/context.rs

@@ -16,6 +16,14 @@ pub struct ResponseContext {
     pub room_id: String,
 }
 
+/// Extract the localpart from a Matrix user ID.
+///
+/// `@sienna:sunbeam.pt` → `sienna`
+pub fn localpart(matrix_user_id: &str) -> &str {
+    let stripped = matrix_user_id.strip_prefix('@').unwrap_or(matrix_user_id);
+    stripped.split(':').next().unwrap_or(stripped)
+}
+
 /// Derive a portable user ID from a Matrix user ID.
 ///
 /// `@sienna:sunbeam.pt` → `sienna@sunbeam.pt`
@@ -53,4 +61,24 @@ mod tests {
             "user@server:8448"
         );
     }
+
+    #[test]
+    fn test_localpart_standard() {
+        assert_eq!(localpart("@sienna:sunbeam.pt"), "sienna");
+    }
+
+    #[test]
+    fn test_localpart_no_at_prefix() {
+        assert_eq!(localpart("sienna:sunbeam.pt"), "sienna");
+    }
+
+    #[test]
+    fn test_localpart_no_colon() {
+        assert_eq!(localpart("@sienna"), "sienna");
+    }
+
+    #[test]
+    fn test_localpart_complex() {
+        assert_eq!(localpart("@user.name:matrix.org"), "user.name");
+    }
 }