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/mod.rs

@@ -53,8 +53,8 @@ pub use types::{
 };
 pub use write::{
     bump_retry_count, migrate_names_from_slugs, migrate_story_ids_to_numeric, name_from_story_id,
-    set_agent, set_blocked, set_depends_on, set_mergemaster_attempted, set_qa_mode,
-    set_retry_count, set_review_hold, write_item,
+    set_agent, set_blocked, set_depends_on, set_epic, set_item_type, set_mergemaster_attempted,
+    set_qa_mode, set_retry_count, set_review_hold, write_item,
 };
 
 #[cfg(test)]