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/http/workflow/bug_ops/bug.rs

@@ -66,23 +66,24 @@ pub fn create_bug_file(
     // Sync depends_on to the typed CRDT register.
     crate::crdt_state::set_depends_on(&bug_id, depends_on.unwrap_or(&[]));
 
+    // Story 933: typed CRDT register for item_type.
+    crate::crdt_state::set_item_type(&bug_id, Some("bug"));
+
     Ok(bug_id)
 }
 
 /// Returns true if the item stem is a bug item.
 ///
 /// Checks the slug-based ID format first (e.g. `"4_bug_login_crash"`), then
-/// falls back to reading `type: bug` from the content store for numeric-only IDs.
+/// consults the typed CRDT `item_type` register for numeric-only IDs (story 933).
 pub(super) fn is_bug_item(stem: &str) -> bool {
     let after_num = stem.trim_start_matches(|c: char| c.is_ascii_digit());
     if after_num.starts_with("_bug_") {
         return true;
     }
-    // Numeric-only ID: check content store front matter.
     if after_num.is_empty() {
-        return crate::db::read_content(stem)
-            .and_then(|c| parse_front_matter(&c).ok())
-            .and_then(|m| m.item_type)
+        return crate::crdt_state::read_item(stem)
+            .and_then(|v| v.item_type().map(str::to_string))
             .map(|t| t == "bug")
             .unwrap_or(false);
     }