wip(929): stage 3 — migrate http/mcp/* off yaml_legacy + introduce yaml_residue marker

Three MCP files touched:

- status_tools.rs (story-status JSON dump): every field with a CRDT
  equivalent now reads from WorkItem (name, agent, blocked, qa_mode,
  retry_count, depends_on, claimed_by, claimed_at) or MergeJob.error
  (merge_failure detail). One field — review_hold — has no CRDT register
  yet (sub-story 932) and is wrapped in `yaml_residue(parse_front_matter(...))`
  so the gap is visible at every code-search.

- qa_tools.rs:
  • tool_approve_qa wraps the legacy `clear_front_matter_field("review_hold")`
    write in `yaml_residue(...)` pending sub-story 932.
  • tool_reject_qa now reads the agent name from the CRDT WorkItem instead
    of parsing front matter on disk.

- story_tools/epic.rs: the entire epic feature (item_type, epic link)
  has no CRDT analog — sub-story 933. Every parse_front_matter call here
  is wrapped in `yaml_residue(...)`.

Also: new identity wrapper `db::yaml_legacy::yaml_residue<T>(v: T) -> T`
that marks a yaml_legacy callsite blocked on a CRDT-register gap. Pure
identity at runtime; the distinctive name makes the residue grep-findable
(`grep -rn yaml_residue`). Sub-stories 932 and 933 enumerate the gaps.

Filed:
- 932: Add CRDT register for review_hold
- 933: Add CRDT registers for the epic mechanism

All 2854 tests pass; fmt + clippy clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/src/db/yaml_legacy.rs

@@ -12,6 +12,23 @@ use serde::Deserialize;
 use std::fs;
 use std::path::Path;
 
+/// Identity wrapper that flags a yaml_legacy callsite blocked on adding a
+/// CRDT register (story 929 residue). Every wrap is a grep-findable marker —
+/// `grep -rn yaml_residue` enumerates every remaining gap — so it stays
+/// visible in every code review.
+///
+/// When the CRDT register lands and the caller is migrated, delete the wrap.
+/// Once every wrap is gone, delete this function and `db::yaml_legacy`
+/// entirely (929 stage 10).
+///
+/// Filed sub-stories enumerate each gap:
+/// - 932: `review_hold` flag (write-side in qa_tools, read-side in
+///   auto_assign).
+/// - 933: epic mechanism — `item_type` and `epic` link fields.
+pub fn yaml_residue<T>(v: T) -> T {
+    v
+}
+
 /// Front-matter fields used by the legacy `parse_front_matter` API.  Mirrors
 /// the original `io::story_metadata::FrontMatter`.
 #[derive(Debug, Default, Deserialize)]