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/lifecycle.rs

@@ -212,27 +212,36 @@ pub fn move_story_to_qa(story_id: &str) -> Result<(), String> {
     .map_err(|e| e.to_string())
 }
 
-/// Move a story from `work/3_qa/` back to `work/2_current/`, clearing `review_hold` and writing notes.
+/// Move a story from `work/3_qa/` back to `work/2_current/`, clearing
+/// `review_hold` (story 932: CRDT register) and appending rejection notes.
 pub fn reject_story_from_qa(story_id: &str, notes: &str) -> Result<(), String> {
-    let notes_owned = notes.to_string();
-    let transform: Box<dyn Fn(&str) -> String> = Box::new(move |content: &str| {
-        let mut result = clear_front_matter_field_in_content(content, "review_hold");
-        if !notes_owned.is_empty() {
-            result =
-                crate::db::yaml_legacy::write_rejection_notes_to_content(&result, &notes_owned);
-        }
-        result
-    });
+    crate::crdt_state::set_review_hold(story_id, false);
 
-    apply_transition(
-        story_id,
-        PipelineEvent::GatesFailed {
-            reason: notes.to_string(),
-        },
-        Some(&*transform),
-    )
-    .map(|_| ())
-    .map_err(|e| e.to_string())
+    if notes.is_empty() {
+        apply_transition(
+            story_id,
+            PipelineEvent::GatesFailed {
+                reason: notes.to_string(),
+            },
+            None,
+        )
+        .map(|_| ())
+        .map_err(|e| e.to_string())
+    } else {
+        let notes_owned = notes.to_string();
+        let transform = move |content: &str| -> String {
+            crate::db::yaml_legacy::write_rejection_notes_to_content(content, &notes_owned)
+        };
+        apply_transition(
+            story_id,
+            PipelineEvent::GatesFailed {
+                reason: notes.to_string(),
+            },
+            Some(&transform),
+        )
+        .map(|_| ())
+        .map_err(|e| e.to_string())
+    }
 }
 
 /// Transition a story to the `Blocked` stage via the state machine.