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/agents/pool/start/spawn.rs

@@ -227,17 +227,9 @@ pub(super) async fn run_agent_spawn(
     }
 
     // Prepend epic context when the story belongs to an epic (AC3, story 880).
-    // 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::yaml_residue(
-            crate::db::yaml_legacy::parse_front_matter(&story_content),
-        )
-        && let Some(ref epic_id) = meta.epic
+    // Story 933: epic linkage is now a typed CRDT register on PipelineItemCrdt.
+    if let Some(view) = crate::crdt_state::read_item(&sid)
+        && let Some(epic_id) = view.epic()
         && let Some(epic_content) = crate::db::read_content(epic_id)
     {
         let block = format!(