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/agents/pool/auto_assign/watchdog/tests/limits_tests.rs

@@ -243,7 +243,7 @@ max_turns = 10
     let story_id = "42_story_runaway";
     let initial = "---\nname: Runaway Story\n---\n# Runaway Story\n";
     crate::db::write_content(story_id, initial);
-    crate::crdt_state::write_item(
+    crate::crdt_state::write_item_str(
         story_id,
         "2_current",
         Some("Runaway Story"),
@@ -274,10 +274,10 @@ max_turns = 10
     let item = crate::crdt_state::read_item(story_id)
         .expect("story must be in CRDT after watchdog termination");
     assert_eq!(
-        item.stage_str(),
-        "2_blocked",
+        item.stage().as_dir(),
+        "blocked",
         "story stage must be 2_blocked after limit termination with max_retries=1 — got: {}",
-        item.stage_str()
+        item.stage().as_dir()
     );
 
     // Sanity: the agent itself is also Failed with the right reason.
@@ -371,7 +371,7 @@ max_turns = 10
 
     let story_id = "story_e_per_session";
     crate::db::write_content(story_id, "---\nname: Per-Session Test\n---\n");
-    crate::crdt_state::write_item(
+    crate::crdt_state::write_item_str(
         story_id,
         "2_current",
         Some("Per-Session Test"),
@@ -416,8 +416,8 @@ max_turns = 10
     let item = crate::crdt_state::read_item(story_id)
         .expect("story must be in CRDT after per-session overrun");
     assert_eq!(
-        item.stage_str(),
-        "2_blocked",
+        item.stage().as_dir(),
+        "blocked",
         "story stage must be 2_blocked after per-session overrun with max_retries=1"
     );
 }
@@ -451,7 +451,7 @@ max_turns = 10
     let initial = "---\nname: Retry Test\n---\n";
     crate::crdt_state::init_for_test();
     crate::db::write_content(story_id, initial);
-    crate::crdt_state::write_item(
+    crate::crdt_state::write_item_str(
         story_id,
         "2_current",
         Some("Retry Test"),
@@ -478,8 +478,8 @@ max_turns = 10
             "after session 1, retry_count should be 1 in CRDT"
         );
         assert_ne!(
-            item.stage_str(),
-            "2_blocked",
+            item.stage().as_dir(),
+            "blocked",
             "story should NOT be blocked after session 1"
         );
     }
@@ -498,8 +498,8 @@ max_turns = 10
             "after session 2, retry_count should be 2 in CRDT"
         );
         assert_ne!(
-            item.stage_str(),
-            "2_blocked",
+            item.stage().as_dir(),
+            "blocked",
             "story should NOT be blocked after session 2"
         );
     }
@@ -513,10 +513,10 @@ max_turns = 10
 
         let item = crate::crdt_state::read_item(story_id).expect("story must be in CRDT");
         assert_eq!(
-            item.stage_str(),
-            "2_blocked",
+            item.stage().as_dir(),
+            "blocked",
             "story must be blocked after session 3 (retry_count=3 >= max_retries=3) — got: {}",
-            item.stage_str()
+            item.stage().as_dir()
         );
         // retry_count resets to 0 on stage transition (Bug 780) — the fact
         // that the story reached 2_blocked proves the retry limit was hit.