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/pipeline/advance/helpers.rs

@@ -66,25 +66,10 @@ pub(super) fn resolve_qa_mode_from_store(
         .unwrap_or(default)
 }
 
-/// Write review_hold to the content store.
+/// Mark a story as held for human review (story 932: CRDT register).
 pub(super) fn write_review_hold_to_store(story_id: &str) {
-    if let Some(contents) = crate::db::read_content(story_id) {
-        let updated = crate::db::yaml_legacy::write_review_hold_in_content(&contents);
-        crate::db::write_content(story_id, &updated);
-        // Also persist to SQLite via shadow write.
-        let stage = crate::pipeline_state::read_typed(story_id)
-            .ok()
-            .flatten()
-            .map(|i| i.stage.dir_name().to_string())
-            .unwrap_or_else(|| "3_qa".to_string());
-        crate::db::write_item_with_content(
-            story_id,
-            &stage,
-            &updated,
-            crate::db::ItemMeta::from_yaml(&updated),
-        );
-    } else {
-        slog_error!("[pipeline] Cannot write review_hold for '{story_id}': no content in store");
+    if !crate::crdt_state::set_review_hold(story_id, true) {
+        slog_error!("[pipeline] Cannot set review_hold for '{story_id}': no CRDT entry");
     }
 }