feat(929): delete db/yaml_legacy.rs entirely — CRDT is the sole source of truth

Final 929 sweep: every YAML-shaped helper is gone. No production code
parses or writes YAML front matter anywhere.

Surface removed:
- db/yaml_legacy.rs (FrontMatter/StoryMetadata structs, parse_front_matter,
  set_front_matter_field, yaml_residue marker) — file deleted.
- ItemMeta::from_yaml — deleted; callers pass typed ItemMeta::named(...) or
  ItemMeta::default() and use typed CRDT setters (set_depends_on,
  set_blocked, set_retry_count, set_agent, set_qa_mode, set_review_hold,
  set_item_type, set_epic, set_mergemaster_attempted) for the rest.
- write_coverage_baseline_to_story_file + read_coverage_percent_from_json —
  the coverage_baseline YAML field was write-only (nothing read it back);
  removed along with its caller in agent_tools/lifecycle.rs.
- update_story_in_file's generic `front_matter` HashMap parameter —
  tool_update_story now intercepts every known field name and routes it
  to a typed CRDT setter; unknown keys are rejected with an explicit error
  pointing at the typed setters. The function only takes user_story /
  description sections now.
- All 117 ItemMeta::from_yaml callsites migrated. Where tests previously
  passed a YAML-shaped content blob and relied on the helper to extract
  name/depends_on/blocked/agent/qa, they now pass:
    write_item_with_content(id, stage, content, ItemMeta::named("Foo"))
    crate::crdt_state::set_depends_on(id, &[...])    // when needed
    crate::crdt_state::set_blocked(id, true)         // when needed
    crate::crdt_state::set_agent(id, Some("..."))    // when needed
- write_story_content + write_story_file (test helper) now take an
  explicit `name: Option<&str>` instead of parsing it from content.
- db::ops::move_item_stage stopped re-parsing YAML on every stage
  transition; metadata is read straight from the CRDT view when mirroring
  the row into SQLite.

New CRDT setters added for symmetry:
- crdt_state::set_name (mirrors set_agent — explicit name updates).

cargo fmt --check, clippy --all-targets -- -D warnings, and the
2830-test suite all pass.

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

server/src/http/mcp/story_tools/criteria.rs

@@ -421,9 +421,7 @@ mod tests {
             "9901_test",
             "2_current",
             "---\nname: Test\n---\n## AC\n- [ ] First\n- [x] Done\n- [ ] Second\n",
-            crate::db::ItemMeta::from_yaml(
-                "---\nname: Test\n---\n## AC\n- [ ] First\n- [x] Done\n- [ ] Second\n",
-            ),
+            crate::db::ItemMeta::named("Test"),
         );
 
         let ctx = test_ctx(tmp.path());
@@ -517,7 +515,7 @@ mod tests {
             "9906_story_persist",
             "2_current",
             "---\nname: Persist\n---\n# Story\n",
-            crate::db::ItemMeta::from_yaml("---\nname: Persist\n---\n# Story\n"),
+            crate::db::ItemMeta::named("Persist"),
         );
 
         let ctx = test_ctx(tmp.path());
@@ -555,7 +553,7 @@ mod tests {
             "9905_story_file_only",
             "2_current",
             story_content,
-            crate::db::ItemMeta::from_yaml(story_content),
+            crate::db::ItemMeta::default(),
         );
 
         let ctx = test_ctx(tmp.path());
@@ -627,9 +625,7 @@ mod tests {
             "9997_empty_branch",
             "2_current",
             "---\nname: Empty Branch Test\n---\n## AC\n- [ ] Implement the feature\n",
-            crate::db::ItemMeta::from_yaml(
-                "---\nname: Empty Branch Test\n---\n## AC\n- [ ] Implement the feature\n",
-            ),
+            crate::db::ItemMeta::named("Empty Branch Test"),
         );
 
         let ctx = test_ctx(tmp.path());
@@ -690,9 +686,7 @@ mod tests {
             "9904_test",
             "2_current",
             "---\nname: Test\n---\n## AC\n- [ ] First criterion\n- [x] Already done\n",
-            crate::db::ItemMeta::from_yaml(
-                "---\nname: Test\n---\n## AC\n- [ ] First criterion\n- [x] Already done\n",
-            ),
+            crate::db::ItemMeta::named("Test"),
         );
 
         let ctx = test_ctx(tmp.path());
@@ -744,9 +738,7 @@ mod tests {
             "9905_test",
             "2_current",
             "---\nname: Test\n---\n## Acceptance Criteria\n- [ ] Keep me\n- [ ] Remove me\n",
-            crate::db::ItemMeta::from_yaml(
-                "---\nname: Test\n---\n## Acceptance Criteria\n- [ ] Keep me\n- [ ] Remove me\n",
-            ),
+            crate::db::ItemMeta::named("Test"),
         );
 
         let ctx = test_ctx(tmp.path());
@@ -768,9 +760,7 @@ mod tests {
             "9906_test",
             "2_current",
             "---\nname: Test\n---\n## Acceptance Criteria\n- [ ] Only one\n",
-            crate::db::ItemMeta::from_yaml(
-                "---\nname: Test\n---\n## Acceptance Criteria\n- [ ] Only one\n",
-            ),
+            crate::db::ItemMeta::named("Test"),
         );
 
         let ctx = test_ctx(tmp.path());