wip(929): stage 4 — migrate agents/pool/* + lifecycle.rs read sides off yaml_legacy

Read-side migrations:
- agents/pool/auto_assign/backlog.rs: depends_on check now reads from
  WorkItem.depends_on() instead of parse_front_matter.
- agents/pool/auto_assign/story_checks.rs: read_story_front_matter_agent
  drops its YAML fallback — post-891 the CRDT entry is reliable, and
  removing the fallback makes the contract honest. The now-unused
  read_story_contents helper goes too.
- agents/pool/start/validation.rs: same shape — YAML fallback removed,
  CRDT register is the only source for agent pinning.
- agents/pool/start/spawn.rs: epic-context injection wraps the
  parse_front_matter call in `yaml_residue(...)` since `meta.epic` has no
  CRDT analog (sub-story 933).
- agents/lifecycle.rs: item_type_from_id (numeric-only ID path) wraps its
  parse_front_matter in `yaml_residue(...)` for the same reason (933).
  The write-side `fields_to_clear_transform` calls in lifecycle.rs are
  left for stage 8, when FS-shadow writes are deleted wholesale.

Test fix:
- start_agent_returns_error_when_front_matter_agent_busy now seeds the
  CRDT entry (write_item with agent="coder-opus") instead of relying on
  parse_front_matter reading the YAML on disk.

Filed earlier:
- 932 (review_hold register) — note: this turns out to be a real class-1
  bug: write_review_hold_to_store still writes YAML but has_review_hold
  reads Stage::Frozen, so the write goes into a void. 932 is the correct
  fix.

All 2861 tests pass; fmt + clippy clean.

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

server/src/agents/pool/start/validation.rs

@@ -60,18 +60,10 @@ pub(super) fn read_front_matter_agent(story_id: &str, agent_name: Option<&str>)
     if agent_name.is_some() {
         return None;
     }
-    // After story 871 the pin lives in the CRDT typed register; fall back
-    // to legacy YAML parsing for stories whose CRDT entry doesn't yet have
-    // the field populated.
-    if let Some(view) = crate::crdt_state::read_item(story_id)
-        && let Some(agent) = view.agent()
-        && !agent.is_empty()
-    {
-        return Some(agent.to_string());
-    }
-    crate::db::read_content(story_id).and_then(|contents| {
-        crate::db::yaml_legacy::parse_front_matter(&contents)
-            .ok()?
-            .agent
-    })
+    // Story 929: the agent pin lives in the CRDT typed register; the
+    // legacy YAML fallback is gone — post-891 every story has its CRDT
+    // entry and any story without one has no pinned agent.
+    crate::crdt_state::read_item(story_id)
+        .and_then(|w| w.agent().map(str::to_string))
+        .filter(|s| !s.is_empty())
 }