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/read.rs

@@ -348,6 +348,11 @@ pub(super) fn extract_item_view(item: &PipelineItemCrdt) -> Option<PipelineItemV
         _ => None,
     };
 
+    let review_hold = match item.review_hold.view() {
+        JsonValue::Bool(b) => Some(b),
+        _ => None,
+    };
+
     Some(PipelineItemView {
         story_id,
         stage,
@@ -361,6 +366,7 @@ pub(super) fn extract_item_view(item: &PipelineItemCrdt) -> Option<PipelineItemV
         merged_at,
         qa_mode,
         mergemaster_attempted,
+        review_hold,
     })
 }