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/crdt_state/write/item.rs

@@ -38,6 +38,28 @@ pub fn set_depends_on(story_id: &str, deps: &[u32]) -> bool {
     true
 }
 
+/// Set the `review_hold` CRDT flag for a pipeline item (sub-story 932).
+///
+/// `true` marks the item as held for human review at a pipeline-stage boundary;
+/// the auto-assigner skips items with this flag.  `false` clears the hold and
+/// is written explicitly (the register is not removed) so the cleared state
+/// survives CRDT replay correctly.
+///
+/// Returns `true` if the item was found and the op was applied, `false` otherwise.
+pub fn set_review_hold(story_id: &str, value: bool) -> bool {
+    let Some(state_mutex) = get_crdt() else {
+        return false;
+    };
+    let Ok(mut state) = state_mutex.lock() else {
+        return false;
+    };
+    let Some(&idx) = state.index.get(story_id) else {
+        return false;
+    };
+    apply_and_persist(&mut state, |s| s.crdt.doc.items[idx].review_hold.set(value));
+    true
+}
+
 /// Set the `mergemaster_attempted` CRDT flag for a pipeline item.
 ///
 /// Passing `true` records that a mergemaster session has been spawned for this
@@ -227,6 +249,7 @@ pub fn write_item(
             "merged_at": merged_at.unwrap_or(0.0),
             "qa_mode": "",
             "mergemaster_attempted": false,
+            "review_hold": false,
         })
         .into();
 
@@ -254,6 +277,7 @@ pub fn write_item(
             item.merged_at.advance_seq(floor);
             item.qa_mode.advance_seq(floor);
             item.mergemaster_attempted.advance_seq(floor);
+            item.review_hold.advance_seq(floor);
         }
 
         // Broadcast a CrdtEvent for the new item.