wip(929): stage 5 — drop FS-based dep checks and qa-mode parser from io/story_metadata

Migrate the last three callers of the FS-scanning dependency helpers to the
CRDT-direct equivalents and delete the dead helpers:

- agents/pool/auto_assign/story_checks.rs: has_unmet_dependencies and
  check_archived_dependencies now wrap check_unmet_deps_crdt /
  check_archived_deps_crdt directly. Tests rewritten to seed the CRDT.
- http/mcp/story_tools/story/update.rs: bug-503 archived-dep warning now
  reads from CRDT instead of scanning 6_archived.
- agents/pool/pipeline/advance/helpers.rs: resolve_qa_mode_from_store is
  CRDT-only (the FS fallback for content-store-empty stories is gone).
- io/story_metadata/parser.rs: resolve_qa_mode_from_content removed.
- io/story_metadata/deps.rs: check_unmet_deps and dep_is_done deleted,
  along with the unused check_unmet_deps_from_list helper.
- io/story_metadata/mod.rs: re-exports trimmed accordingly.

check_archived_deps_from_list survives because story-creation still calls
it before the CRDT entry exists (used from story_tools/story/create.rs).

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

server/src/agents/pool/pipeline/advance/helpers.rs

@@ -52,29 +52,18 @@ pub(crate) fn spawn_pipeline_advance(
 
 /// Resolve QA mode for a story.
 ///
-/// Checks the typed `qa_mode` CRDT register first.  If the register holds a
-/// recognised value, returns it immediately without touching the content store.
-/// Otherwise reads the story content and falls back to YAML front-matter
-/// parsing via [`crate::io::story_metadata::resolve_qa_mode_from_content`].
+/// Story 929: reads the typed `qa_mode` register; the legacy YAML fallback
+/// is gone (the CRDT register is the only source).
 pub(super) fn resolve_qa_mode_from_store(
     _project_root: &Path,
     story_id: &str,
     default: crate::io::story_metadata::QaMode,
 ) -> crate::io::story_metadata::QaMode {
-    // CRDT register is the authoritative source; check it before the content store.
-    if let Some(view) = crate::crdt_state::read_item(story_id)
-        && let Some(s) = view.qa_mode()
-        && let Some(mode) = crate::io::story_metadata::QaMode::from_str(s)
-    {
-        return mode;
-    }
-    // Fall back to YAML front matter for backward compatibility.
-    if let Some(contents) = crate::db::read_content(story_id) {
-        return crate::io::story_metadata::resolve_qa_mode_from_content(
-            story_id, &contents, default,
-        );
-    }
-    default
+    crate::crdt_state::read_item(story_id)
+        .and_then(|view| view.qa_mode().map(str::to_string))
+        .as_deref()
+        .and_then(crate::io::story_metadata::QaMode::from_str)
+        .unwrap_or(default)
 }
 
 /// Write review_hold to the content store.