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/http/mcp/qa_tools.rs

@@ -54,17 +54,8 @@ pub(super) async fn tool_approve_qa(args: &Value, ctx: &AppContext) -> Result<St
 
     let project_root = ctx.services.agents.get_project_root(&ctx.state)?;
 
-    // Clear review_hold before moving.
-    // review_hold has no CRDT register yet — story 932. Marked with
-    // `yaml_residue` so the gap is grep-findable.
-    let qa_path = project_root
-        .join(".huskies/work/3_qa")
-        .join(format!("{story_id}.md"));
-    if qa_path.exists() {
-        let _ = crate::db::yaml_legacy::yaml_residue(
-            crate::db::yaml_legacy::clear_front_matter_field(&qa_path, "review_hold"),
-        );
-    }
+    // Clear review_hold before moving (story 932: CRDT register).
+    crate::crdt_state::set_review_hold(story_id, false);
 
     if is_spike(story_id) {
         // Spikes skip the merge stage entirely: merge the feature branch to master