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/chat/commands/depends.rs

@@ -166,6 +166,7 @@ mod tests {
             "1_backlog",
             "9912_story_foo.md",
             "---\nname: Foo\n---\n",
+            None,
         );
         let output = depends_cmd_with_root(tmp.path(), "9912 abc").unwrap();
         assert!(
@@ -182,6 +183,7 @@ mod tests {
             "1_backlog",
             "9910_story_foo.md",
             "---\nname: Foo\n---\n",
+            None,
         );
         let output = depends_cmd_with_root(tmp.path(), "9910 477 478").unwrap();
         assert!(
@@ -212,6 +214,7 @@ mod tests {
             "2_current",
             "9911_story_bar.md",
             "---\nname: Bar\n---\n",
+            None,
         );
         // Pre-seed CRDT with deps so we can verify clearing.
         crate::crdt_state::set_depends_on("9911_story_bar", &[477]);
@@ -248,6 +251,7 @@ mod tests {
             "1_backlog",
             "8790_story_chat_dep.md",
             "---\nname: Chat Dep\n---\n",
+            None,
         );
 
         let out = depends_cmd_with_root(tmp.path(), "8790 500 501").unwrap();
@@ -282,6 +286,7 @@ mod tests {
             "1_backlog",
             "9920_story_scr.md",
             "---\nname: SCR\n---\n",
+            None,
         );
 
         // Set to [1, 2, 3].
@@ -322,6 +327,7 @@ mod tests {
             "3_qa",
             "9913_story_inqa.md",
             "---\nname: In QA\n---\n",
+            None,
         );
         let output = depends_cmd_with_root(tmp.path(), "9913 100").unwrap();
         assert!(