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/service/work_item/freeze.rs

@@ -79,7 +79,7 @@ mod tests {
             story_id,
             "2_current",
             "---\nname: Freeze Service Test\n---\n",
-            crate::db::ItemMeta::from_yaml("---\nname: Freeze Service Test\n---\n"),
+            crate::db::ItemMeta::named("Freeze Service Test"),
         );
 
         let result = freeze(story_id);
@@ -106,7 +106,7 @@ mod tests {
             story_id,
             "2_current",
             "---\nname: Already Frozen\n---\n",
-            crate::db::ItemMeta::from_yaml("---\nname: Already Frozen\n---\n"),
+            crate::db::ItemMeta::named("Already Frozen"),
         );
 
         freeze(story_id).expect("first freeze should succeed");
@@ -126,7 +126,7 @@ mod tests {
             story_id,
             "2_current",
             "---\nname: Unfreeze Service Test\n---\n",
-            crate::db::ItemMeta::from_yaml("---\nname: Unfreeze Service Test\n---\n"),
+            crate::db::ItemMeta::named("Unfreeze Service Test"),
         );
 
         freeze(story_id).expect("freeze should succeed");
@@ -155,7 +155,7 @@ mod tests {
             story_id,
             "2_current",
             "---\nname: Not Frozen\n---\n",
-            crate::db::ItemMeta::from_yaml("---\nname: Not Frozen\n---\n"),
+            crate::db::ItemMeta::named("Not Frozen"),
         );
 
         let result = unfreeze(story_id);
@@ -179,7 +179,7 @@ mod tests {
             story_a,
             "2_current",
             "---\nname: Regression Chat Path\n---\n",
-            crate::db::ItemMeta::from_yaml("---\nname: Regression Chat Path\n---\n"),
+            crate::db::ItemMeta::named("Regression Chat Path"),
         );
 
         // Story B simulates the MCP tool path.
@@ -188,7 +188,7 @@ mod tests {
             story_b,
             "2_current",
             "---\nname: Regression MCP Path\n---\n",
-            crate::db::ItemMeta::from_yaml("---\nname: Regression MCP Path\n---\n"),
+            crate::db::ItemMeta::named("Regression MCP Path"),
         );
 
         // Both paths call service::work_item::freeze().