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/agent_tools/lifecycle.rs

@@ -3,7 +3,6 @@
 use serde_json::{Value, json};
 
 use crate::http::context::AppContext;
-use crate::slog_warn;
 
 use super::worktree::get_worktree_commits;
 
@@ -32,16 +31,10 @@ pub(crate) async fn tool_start_agent(args: &Value, ctx: &AppContext) -> Result<S
             .await?
     };
 
-    // Snapshot coverage baseline from the most recent coverage report (best-effort).
-    if let Some(pct) = read_coverage_percent_from_json(&project_root)
-        && let Err(e) = crate::http::workflow::write_coverage_baseline_to_story_file(
-            &project_root,
-            story_id,
-            pct,
-        )
-    {
-        slog_warn!("[start_agent] Could not write coverage baseline to story file: {e}");
-    }
+    // Story 929: the legacy "snapshot coverage baseline into story YAML"
+    // hook is gone — the field was write-only (nothing read it back) and
+    // the per-run report at `.huskies/coverage/server.json` is the
+    // authoritative source for downstream tools.
 
     serde_json::to_string_pretty(&json!({
         "story_id": info.story_id,
@@ -53,21 +46,6 @@ pub(crate) async fn tool_start_agent(args: &Value, ctx: &AppContext) -> Result<S
     .map_err(|e| format!("Serialization error: {e}"))
 }
 
-/// Try to read the overall line coverage percentage from the llvm-cov JSON report.
-///
-/// Expects the file at `{project_root}/.huskies/coverage/server.json`.
-pub(crate) fn read_coverage_percent_from_json(project_root: &std::path::Path) -> Option<f64> {
-    let path = project_root
-        .join(".huskies")
-        .join("coverage")
-        .join("server.json");
-    let contents = std::fs::read_to_string(&path).ok()?;
-    let json: Value = serde_json::from_str(&contents).ok()?;
-    // cargo llvm-cov --json format: data[0].totals.lines.percent
-    json.pointer("/data/0/totals/lines/percent")
-        .and_then(|v| v.as_f64())
-}
-
 pub(crate) async fn tool_stop_agent(args: &Value, ctx: &AppContext) -> Result<String, String> {
     let story_id = args
         .get("story_id")
@@ -334,25 +312,4 @@ stage = "coder"
         // completion key present (null for agents that didn't call report_completion)
         assert!(parsed.get("completion").is_some());
     }
-
-    #[test]
-    fn read_coverage_percent_from_json_parses_llvm_cov_format() {
-        use std::fs;
-        let tmp = tempfile::tempdir().unwrap();
-        let cov_dir = tmp.path().join(".huskies/coverage");
-        fs::create_dir_all(&cov_dir).unwrap();
-        let json_content =
-            r#"{"data":[{"totals":{"lines":{"count":100,"covered":78,"percent":78.0}}}]}"#;
-        fs::write(cov_dir.join("server.json"), json_content).unwrap();
-
-        let pct = read_coverage_percent_from_json(tmp.path());
-        assert_eq!(pct, Some(78.0));
-    }
-
-    #[test]
-    fn read_coverage_percent_from_json_returns_none_when_absent() {
-        let tmp = tempfile::tempdir().unwrap();
-        let pct = read_coverage_percent_from_json(tmp.path());
-        assert!(pct.is_none());
-    }
 }