feat(932): add review_hold CRDT register + migrate callers off yaml_legacy

review_hold is now a typed bool register on PipelineItemCrdt alongside
blocked / mergemaster_attempted. Exposed via the typed setter
`crdt_state::set_review_hold(story_id, value)` and the
`WorkItem::review_hold()` accessor. Replaces the legacy
`review_hold: true` YAML front-matter field.

Migrated callers:
- http/mcp/qa_tools.rs::tool_approve_qa  — clear via set_review_hold(false)
- agents/lifecycle.rs::reject_story_from_qa  — clear via set_review_hold(false)
- agents/pool/pipeline/advance/helpers.rs::write_review_hold_to_store
  — set via set_review_hold(true), no more content rewrite
- agents/pool/auto_assign/reconcile.rs (two callsites) — set via
  set_review_hold(true) instead of FS YAML write
- agents/pool/auto_assign/story_checks.rs::has_review_hold — reads the
  typed register instead of conflating with Stage::Frozen (real bug fix:
  the legacy implementation returned `stage.is_frozen()`, which made
  the auto-assigner treat *every* held-for-review item as frozen even
  when it wasn't actually parked at the freeze stage).

Dead yaml_legacy helpers removed:
- write_review_hold(path), write_review_hold_in_content(content)
- clear_front_matter_field(path) — last caller was the qa_tools wrap

The yaml_residue marker doc now only mentions 933; the 932 line is gone.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -21,17 +21,15 @@ pub(super) fn read_story_front_matter_agent(
         .filter(|s| !s.is_empty())
 }
 
-/// Return `true` if the story is in the `Frozen` pipeline stage.
+/// Return `true` if the story has its `review_hold` CRDT register set.
 ///
-/// In the typed CRDT model, `Frozen` is the authoritative representation of
-/// stories that are held for human review (replacing the legacy
-/// `review_hold: true` YAML front-matter field). The typed stage register is
-/// the only source consulted — stale YAML is ignored.
+/// Sub-story 932: `review_hold` is now a dedicated CRDT register on
+/// `PipelineItemCrdt`, distinct from `Stage::Frozen`. The auto-assigner uses
+/// this to keep human-QA items / spikes parked after gates pass until a
+/// reviewer explicitly clears the hold (e.g. via `tool_approve_qa`).
 pub(super) fn has_review_hold(_project_root: &Path, _stage_dir: &str, story_id: &str) -> bool {
-    crate::pipeline_state::read_typed(story_id)
-        .ok()
-        .flatten()
-        .map(|item| item.stage.is_frozen())
+    crate::crdt_state::read_item(story_id)
+        .map(|w| w.review_hold())
         .unwrap_or(false)
 }
 
@@ -138,29 +136,42 @@ mod tests {
     // ── has_review_hold ───────────────────────────────────────────────────────
 
     #[test]
-    fn has_review_hold_returns_true_when_frozen() {
+    fn has_review_hold_returns_true_when_flag_set() {
         crate::crdt_state::init_for_test();
         crate::db::ensure_content_store();
         let tmp = tempfile::tempdir().unwrap();
-        crate::db::write_item_with_content(
-            "890_spike_frozen",
-            "7_frozen",
-            "---\nname: Frozen Spike\n---\n# Spike\n",
-            crate::db::ItemMeta::named("Frozen Spike"),
+        crate::crdt_state::write_item(
+            "890_spike_held",
+            "3_qa",
+            Some("Held Spike"),
+            None,
+            None,
+            None,
+            None,
+            None,
+            None,
+            None,
         );
-        assert!(has_review_hold(tmp.path(), "3_qa", "890_spike_frozen"));
+        crate::crdt_state::set_review_hold("890_spike_held", true);
+        assert!(has_review_hold(tmp.path(), "3_qa", "890_spike_held"));
     }
 
     #[test]
-    fn has_review_hold_returns_false_for_qa_stage() {
+    fn has_review_hold_returns_false_when_flag_unset() {
         crate::crdt_state::init_for_test();
         crate::db::ensure_content_store();
         let tmp = tempfile::tempdir().unwrap();
-        crate::db::write_item_with_content(
+        crate::crdt_state::write_item(
             "890_spike_active_qa",
             "3_qa",
-            "---\nname: Active QA Spike\n---\n# Spike\n",
-            crate::db::ItemMeta::named("Active QA Spike"),
+            Some("Active QA Spike"),
+            None,
+            None,
+            None,
+            None,
+            None,
+            None,
+            None,
         );
         assert!(!has_review_hold(tmp.path(), "3_qa", "890_spike_active_qa"));
     }