feat(933): add item_type + epic CRDT registers + migrate epic mechanism

Replaces the YAML-only `type: epic` / `epic: <id>` front-matter fields with
typed CRDT registers on PipelineItemCrdt. The epic-mechanism MCP tools
(`tool_list_epics`, `tool_show_epic`), the epic-context injection in agent
spawn, and the type-classifier helpers (`item_type_from_id`, `is_bug_item`,
`is_refactor_item`) now all read from the CRDT.

Schema:
- PipelineItemCrdt: `item_type: LwwRegisterCrdt<String>` and
  `epic: LwwRegisterCrdt<String>` registers.
- WorkItem: typed `item_type()` and `epic()` accessors returning `Option<&str>`.
- crdt_state::set_item_type(story_id, Option<&str>) and
  crdt_state::set_epic(story_id, Option<&str>) typed setters.

Write paths populate the new registers:
- create_story_file / create_bug_file / create_spike_file /
  create_refactor_file / create_epic_file — each calls set_item_type after
  write_story_content.
- tool_update_story intercepts `epic` and `type` fields and routes them to
  the typed setters (same pattern as qa / depends_on).

Read paths migrated off yaml_legacy:
- http/mcp/story_tools/epic.rs: tool_list_epics + tool_show_epic.
- agents/lifecycle.rs::item_type_from_id (numeric-only IDs).
- agents/pool/start/spawn.rs epic-context injection.
- http/workflow/bug_ops/bug.rs::is_bug_item, refactor.rs::is_refactor_item.
- http/workflow/pipeline.rs::load_pipeline_state — review_hold/qa/epic_id
  all come from the CRDT now; only merge_failure is still YAML (sweep in
  929 stage 10).

All `yaml_residue(...)` wraps for item_type / epic are removed; the
remaining residue marker doc no longer references 933.

cargo fmt --check, clippy --all-targets -- -D warnings, and the 2857-test
suite all pass.

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

server/src/crdt_state/write/item.rs

@@ -38,6 +38,49 @@ pub fn set_depends_on(story_id: &str, deps: &[u32]) -> bool {
     true
 }
 
+/// Set the `item_type` CRDT register for a pipeline item (sub-story 933).
+///
+/// `Some(t)` writes the type string (e.g. `"story"`, `"epic"`, `"bug"`).
+/// `None` clears the register to an empty string, which means "use the
+/// id-prefix heuristic" (see `item_type_from_id`).
+///
+/// Returns `true` if the item was found and the op was applied, `false` otherwise.
+pub fn set_item_type(story_id: &str, item_type: Option<&str>) -> bool {
+    let Some(state_mutex) = get_crdt() else {
+        return false;
+    };
+    let Ok(mut state) = state_mutex.lock() else {
+        return false;
+    };
+    let Some(&idx) = state.index.get(story_id) else {
+        return false;
+    };
+    let value = item_type.unwrap_or("").to_string();
+    apply_and_persist(&mut state, |s| s.crdt.doc.items[idx].item_type.set(value));
+    true
+}
+
+/// Set the `epic` CRDT register for a pipeline item (sub-story 933).
+///
+/// `Some(epic_id)` links the item to its parent epic.
+/// `None` clears the register to an empty string (no epic membership).
+///
+/// Returns `true` if the item was found and the op was applied, `false` otherwise.
+pub fn set_epic(story_id: &str, epic_id: Option<&str>) -> bool {
+    let Some(state_mutex) = get_crdt() else {
+        return false;
+    };
+    let Ok(mut state) = state_mutex.lock() else {
+        return false;
+    };
+    let Some(&idx) = state.index.get(story_id) else {
+        return false;
+    };
+    let value = epic_id.unwrap_or("").to_string();
+    apply_and_persist(&mut state, |s| s.crdt.doc.items[idx].epic.set(value));
+    true
+}
+
 /// Set the `review_hold` CRDT flag for a pipeline item (sub-story 932).
 ///
 /// `true` marks the item as held for human review at a pipeline-stage boundary;
@@ -250,6 +293,8 @@ pub fn write_item(
             "qa_mode": "",
             "mergemaster_attempted": false,
             "review_hold": false,
+            "item_type": "",
+            "epic": "",
         })
         .into();
 
@@ -278,6 +323,8 @@ pub fn write_item(
             item.qa_mode.advance_seq(floor);
             item.mergemaster_attempted.advance_seq(floor);
             item.review_hold.advance_seq(floor);
+            item.item_type.advance_seq(floor);
+            item.epic.advance_seq(floor);
         }
 
         // Broadcast a CrdtEvent for the new item.