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/lifecycle.rs

@@ -32,9 +32,13 @@ pub(crate) fn item_type_from_id(item_id: &str) -> &'static str {
         return "refactor";
     }
     // Numeric-only ID: check content store front matter for explicit type.
+    // `item_type` has no CRDT register yet (story 933 — epic mechanism).
+    // Marked with yaml_residue so the gap is grep-findable.
     if after_num.is_empty()
         && let Some(content) = crate::db::read_content(item_id)
-        && let Ok(meta) = crate::db::yaml_legacy::parse_front_matter(&content)
+        && let Ok(meta) = crate::db::yaml_legacy::yaml_residue(
+            crate::db::yaml_legacy::parse_front_matter(&content),
+        )
         && let Some(t) = meta.item_type.as_deref()
     {
         return match t {

server/src/agents/pool/auto_assign/backlog.rs

@@ -24,16 +24,12 @@ impl AgentPool {
     /// logged so the user can see the promotion was triggered by an archived dep, not
     /// a clean completion.
     pub(super) fn promote_ready_backlog_stories(&self, project_root: &Path) {
-        use crate::db::yaml_legacy::parse_front_matter;
-
         let items = scan_stage_items(project_root, "1_backlog");
         for story_id in &items {
-            // Only promote stories that explicitly declare dependencies.
-            let contents = crate::db::read_content(story_id);
-            let has_deps = contents
-                .and_then(|c| parse_front_matter(&c).ok())
-                .and_then(|m| m.depends_on)
-                .map(|d| !d.is_empty())
+            // Only promote stories that explicitly declare dependencies
+            // (story 929: read from the CRDT register, not YAML).
+            let has_deps = crate::crdt_state::read_item(story_id)
+                .map(|w| !w.depends_on().is_empty())
                 .unwrap_or(false);
             if !has_deps {
                 continue;

server/src/agents/pool/auto_assign/story_checks.rs

@@ -2,11 +2,6 @@
 
 use std::path::Path;
 
-/// Read story contents from the DB content store (CRDT-backed).
-fn read_story_contents(_project_root: &Path, story_id: &str) -> Option<String> {
-    crate::db::read_content(story_id)
-}
-
 /// Read the optional `agent:` pin for a story.
 ///
 /// After story 871 the agent assignment lives in the CRDT typed register
@@ -14,19 +9,16 @@ fn read_story_contents(_project_root: &Path, story_id: &str) -> Option<String> {
 /// first; falling back to legacy YAML parsing keeps behaviour intact for any
 /// stories whose CRDT entry doesn't yet have the field set.
 pub(super) fn read_story_front_matter_agent(
-    project_root: &Path,
+    _project_root: &Path,
     _stage_dir: &str,
     story_id: &str,
 ) -> Option<String> {
-    if let Some(view) = crate::crdt_state::read_item(story_id)
-        && let Some(agent) = view.agent()
-        && !agent.is_empty()
-    {
-        return Some(agent.to_string());
-    }
-    use crate::db::yaml_legacy::parse_front_matter;
-    let contents = read_story_contents(project_root, story_id)?;
-    parse_front_matter(&contents).ok()?.agent
+    // Story 929: agent name comes from the CRDT register. The previous
+    // YAML fallback is gone — post-891 every story has its CRDT entry,
+    // and any story without one is treated as having no pinned agent.
+    crate::crdt_state::read_item(story_id)
+        .and_then(|w| w.agent().map(str::to_string))
+        .filter(|s| !s.is_empty())
 }
 
 /// Return `true` if the story is in the `Frozen` pipeline stage.

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

@@ -230,8 +230,13 @@ pub(super) async fn run_agent_spawn(
     // Read the story's front matter to find the epic ID, then load the epic's
     // content and prepend it to the system prompt so the agent treats it as
     // authoritative context.
+    //
+    // Epic linkage has no CRDT register yet (story 933) — wrap the parse in
+    // `yaml_residue` so the gap is grep-findable.
     if let Some(story_content) = crate::db::read_content(&sid)
-        && let Ok(meta) = crate::db::yaml_legacy::parse_front_matter(&story_content)
+        && let Ok(meta) = crate::db::yaml_legacy::yaml_residue(
+            crate::db::yaml_legacy::parse_front_matter(&story_content),
+        )
         && let Some(ref epic_id) = meta.epic
         && let Some(epic_content) = crate::db::read_content(epic_id)
     {

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

@@ -281,6 +281,20 @@ stage = "coder"
     std::fs::write(current.join("368_story_test.md"), story_content).unwrap();
     crate::db::ensure_content_store();
     crate::db::write_content("368_story_test", story_content);
+    // Story 929: agent pin comes from the CRDT register, not YAML. Seed it.
+    crate::crdt_state::init_for_test();
+    crate::crdt_state::write_item(
+        "368_story_test",
+        "2_current",
+        Some("Test Story"),
+        Some("coder-opus"),
+        None,
+        None,
+        None,
+        None,
+        None,
+        None,
+    );
 
     let pool = AgentPool::new_test(3011);
     // Preferred agent is busy — should NOT fall back to coder-sonnet.

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())
 }