feat(934): typed Stage enum replaces directory-string state model

The state machine's `Stage` enum becomes the source of truth for pipeline
state. Six stages of work land together:

  1. Clean wire vocabulary (`coding`, `merge`, `merge_failure`, ...) replaces
     legacy directory-style strings (`2_current`, `4_merge`, ...) on the wire.
     `Stage::from_dir` accepted both during deployment; new writes always
     emit the clean form via `stage_dir_name`. Lexicographic `dir >= "5_done"`
     checks in lifecycle.rs become typed `matches!` checks since the new
     vocabulary doesn't sort in pipeline order.
  2. `crdt_state::write_item` takes typed `&Stage`, serialising via
     `stage_dir_name` at the CRDT boundary. `#[cfg(test)] write_item_str`
     parses legacy strings for test fixtures.
  3. `WorkItem::stage()` returns typed `crdt_state::Stage`; `stage_str()`
     is gone from the public API. Projection dispatches on the typed enum.
  4. `frozen` becomes an orthogonal CRDT register. `Stage::Frozen` and
     `PipelineEvent::Freeze`/`Unfreeze` are removed; `transition_to_frozen`/
     `unfrozen` set the flag directly without touching the stage register.
  5. Watcher sweep and `tool_update_story`'s `blocked` setter route through
     `apply_transition` so the typed transition table validates every
     stage change. `update_story` gains a `frozen` field for symmetry.
  6. One-shot startup migration rewrites pre-934 directory-style stage
     registers (and sets `frozen=true` on items previously at `7_frozen`).
     `Stage::from_dir` drops legacy aliases. The db boundary keeps a small
     normaliser so callers with legacy strings (MCP, tests) still work.

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

server/src/http/mcp/merge_tools.rs

@@ -17,21 +17,18 @@ pub(super) async fn tool_merge_agent_work(
     // Check CRDT stage before attempting merge — if already done or archived,
     // return success immediately to avoid spurious error notifications.
     if let Some(item) = crate::crdt_state::read_item(story_id)
-        && crate::pipeline_state::Stage::from_dir(item.stage_str()).is_some_and(|s| {
-            matches!(
-                s,
-                crate::pipeline_state::Stage::Done { .. }
-                    | crate::pipeline_state::Stage::Archived { .. }
-            )
-        })
+        && matches!(
+            item.stage(),
+            crate::crdt_state::Stage::Done | crate::crdt_state::Stage::Archived
+        )
     {
+        let stage_name = item.stage().as_dir().to_string();
         return serde_json::to_string_pretty(&json!({
             "story_id": story_id,
             "status": "completed",
             "success": true,
             "message": format!(
-                "Story '{}' is already in '{}' — no merge needed.",
-                story_id, item.stage_str()
+                "Story '{story_id}' is already in '{stage_name}' — no merge needed.",
             ),
         }))
         .map_err(|e| format!("Serialization error: {e}"));
@@ -283,7 +280,7 @@ mod tests {
     #[tokio::test]
     async fn tool_merge_agent_work_already_done_returns_success() {
         crate::crdt_state::init_for_test();
-        crate::crdt_state::write_item(
+        crate::crdt_state::write_item_str(
             "99_story_already_done",
             "5_done",
             Some("Already done story"),
@@ -304,13 +301,13 @@ mod tests {
         let v: serde_json::Value = serde_json::from_str(&body).unwrap();
         assert_eq!(v["status"], "completed");
         assert_eq!(v["success"], true);
-        assert!(v["message"].as_str().unwrap().contains("5_done"));
+        assert!(v["message"].as_str().unwrap().contains("done"));
     }
 
     #[tokio::test]
     async fn tool_merge_agent_work_already_archived_returns_success() {
         crate::crdt_state::init_for_test();
-        crate::crdt_state::write_item(
+        crate::crdt_state::write_item_str(
             "98_story_already_archived",
             "6_archived",
             Some("Already archived story"),
@@ -331,7 +328,7 @@ mod tests {
         let v: serde_json::Value = serde_json::from_str(&body).unwrap();
         assert_eq!(v["status"], "completed");
         assert_eq!(v["success"], true);
-        assert!(v["message"].as_str().unwrap().contains("6_archived"));
+        assert!(v["message"].as_str().unwrap().contains("archived"));
     }
 
     #[tokio::test]