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/unblock.rs

@@ -181,6 +181,7 @@ mod tests {
             "2_current",
             "42_story_test.md",
             "---\nname: Test Story\nretry_count: 2\n---\n# Story\n",
+            None,
         );
         let output = unblock_cmd_with_root(tmp.path(), "42").unwrap();
         assert!(
@@ -200,6 +201,7 @@ mod tests {
             "2_blocked",
             "9903_story_stuck.md",
             "---\nname: Stuck Story\nblocked: true\nretry_count: 5\n---\n# Story\n",
+            None,
         );
         // Seed the story in the CRDT in 2_blocked stage so the typed
         // Blocked → Coding transition fires and clears `blocked` properly.
@@ -267,7 +269,7 @@ mod tests {
             story_id,
             stage,
             body,
-            crate::db::ItemMeta::from_yaml(body),
+            crate::db::ItemMeta::named("Stuck Story"),
         );
         // Seed CRDT registers: blocked=true, retry_count=5, with a name so the
         // response can echo it back instead of falling through to the raw id.
@@ -312,14 +314,15 @@ mod tests {
     #[test]
     fn unblock_command_finds_story_in_any_stage() {
         let tmp = tempfile::TempDir::new().unwrap();
-        // Use a high story number (9901) to avoid collisions with other tests in the
-        // global content store.
         write_story_file(
             tmp.path(),
             "3_qa",
             "9901_story_in_qa.md",
-            "---\nname: In QA\nblocked: true\nretry_count: 3\n---\n# Story\n",
+            "# Story\n",
+            Some("In QA"),
         );
+        crate::crdt_state::set_blocked("9901_story_in_qa", true);
+        crate::crdt_state::set_retry_count("9901_story_in_qa", 3);
 
         let output = unblock_cmd_with_root(tmp.path(), "9901").unwrap();
         assert!(
@@ -338,6 +341,7 @@ mod tests {
             "1_backlog",
             "9902_story_blocked_one.md",
             "---\nname: Blocked One\nblocked: true\nretry_count: 2\n---\n",
+            None,
         );
 
         let output = unblock_cmd_with_root(tmp.path(), "9902").unwrap();