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/mod.rs

@@ -54,7 +54,7 @@ pub use types::{
 pub use write::{
     bump_retry_count, migrate_names_from_slugs, migrate_story_ids_to_numeric, name_from_story_id,
     set_agent, set_blocked, set_depends_on, set_mergemaster_attempted, set_qa_mode,
-    set_retry_count, write_item,
+    set_retry_count, set_review_hold, write_item,
 };
 
 #[cfg(test)]