wip(929): stage 2 — migrate chat/transport/matrix/* off yaml_legacy

delete.rs, start.rs, assign.rs all looked up the story name by reading
the content from disk/store and parsing the front matter. Replaced with
`crdt_state::read_item(&story_id).and_then(|w| w.name())`. Each callsite's
fallback chain ("CRDT → content store → filesystem") still locates the
story_id; only the name extraction moved off YAML.

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

server/src/chat/transport/matrix/start.rs

@@ -80,7 +80,7 @@ pub async fn handle_start(
     agents: &AgentPool,
 ) -> String {
     // Find the story by numeric prefix: CRDT → content store → filesystem.
-    let (story_id, _stage_dir, _path, content) =
+    let (story_id, _stage_dir, _path, _content) =
         match crate::chat::lookup::find_story_by_number(project_root, story_number) {
             Some(found) => found,
             None => {
@@ -88,12 +88,9 @@ pub async fn handle_start(
             }
         };
 
-    let story_name = content
-        .and_then(|contents| {
-            crate::db::yaml_legacy::parse_front_matter(&contents)
-                .ok()
-                .and_then(|m| m.name)
-        })
+    // Story name comes from the CRDT name register (story 929).
+    let story_name = crate::crdt_state::read_item(&story_id)
+        .and_then(|w| w.name().map(str::to_string))
         .unwrap_or_else(|| story_id.clone());
 
     // Resolve agent name: try "coder-{hint}" first, then the hint as-is.