fix: read agent pin from CRDT register, not just YAML front matter

After story 871 the `agent` pin lives in the typed CRDT register
(`PipelineItemView.agent`), not the YAML front matter — the YAML
mutation was removed at the same time. Both spawn-resolution paths
(`auto_assign::story_checks::read_story_front_matter_agent` and
`start::validation::read_front_matter_agent`) still read only YAML
via parse_front_matter, which returns None for any story whose pin
was set via the post-871 typed setter. The spawn then falls back to
"first available coder," silently downgrading opus-pinned stories to
the first available sonnet — which is why 855/864/866 kept hitting the
80-turn watchdog limit despite the user's explicit opus pin.

Now: both paths consult `crdt_state::read_item()` first and use
`view.agent` if non-empty. YAML parsing remains as a fallback so older
stories whose CRDT entry doesn't yet have the field still resolve.

Adds a regression test that seeds an item with empty YAML, sets the
typed CRDT register via `set_agent`, and asserts
`read_story_front_matter_agent` returns the CRDT value.

server/src/agents/pool/auto_assign/story_checks.rs

@@ -7,15 +7,23 @@ fn read_story_contents(_project_root: &Path, story_id: &str) -> Option<String> {
     crate::db::read_content(story_id)
 }
 
-/// Read the optional `agent:` field from the front matter of a story file.
+/// Read the optional `agent:` pin for a story.
 ///
-/// Returns `Some(agent_name)` if the front matter specifies an agent, or `None`
-/// if the field is absent or the file cannot be read / parsed.
+/// After story 871 the agent assignment lives in the CRDT typed register
+/// (`PipelineItemView.agent`), not the YAML front matter. We check the CRDT
+/// first; falling back to legacy YAML parsing keeps behaviour intact for any
+/// stories whose CRDT entry doesn't yet have the field set.
 pub(super) fn read_story_front_matter_agent(
     project_root: &Path,
     _stage_dir: &str,
     story_id: &str,
 ) -> Option<String> {
+    if let Some(view) = crate::crdt_state::read_item(story_id)
+        && let Some(agent) = view.agent.as_ref()
+        && !agent.is_empty()
+    {
+        return Some(agent.clone());
+    }
     use crate::io::story_metadata::parse_front_matter;
     let contents = read_story_contents(project_root, story_id)?;
     parse_front_matter(&contents).ok()?.agent
@@ -293,4 +301,29 @@ mod tests {
             check_archived_dependencies(tmp.path(), "1_backlog", "503_story_waiting");
         assert!(archived_deps.is_empty());
     }
+
+    #[test]
+    fn read_story_front_matter_agent_prefers_crdt_typed_register_over_yaml() {
+        // Regression: after story 871 the agent pin lives in the CRDT typed
+        // register, not the YAML front matter. read_story_front_matter_agent
+        // must consult the register first so auto-assign honours the pin.
+        let tmp = tempfile::tempdir().unwrap();
+        crate::db::ensure_content_store();
+
+        // Seed a story whose YAML has NO agent field.
+        crate::db::write_item_with_content(
+            "9971_story_pin_in_crdt",
+            "2_current",
+            "---\nname: Pin In CRDT\n---\n",
+        );
+
+        // Set the typed CRDT register (this is the path 871's migration uses).
+        let written =
+            crate::crdt_state::set_agent("9971_story_pin_in_crdt", Some("coder-opus"));
+        assert!(written, "set_agent should succeed for an existing item");
+
+        // The reader must return the CRDT register value, not None.
+        let agent = read_story_front_matter_agent(tmp.path(), "2_current", "9971_story_pin_in_crdt");
+        assert_eq!(agent.as_deref(), Some("coder-opus"));
+    }
 }