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/service/notifications/format.rs

@@ -19,7 +19,6 @@ pub fn stage_display_name(stage: &str) -> &'static str {
         Some(Stage::Done { .. }) => "Done",
         Some(Stage::Archived { .. }) => "Archived",
         Some(Stage::MergeFailure { .. }) => "MergeFailure",
-        Some(Stage::Frozen { .. }) => "Frozen",
         None => "Unknown",
     }
 }
@@ -185,12 +184,12 @@ mod tests {
 
     #[test]
     fn stage_display_name_maps_all_known_stages() {
-        assert_eq!(stage_display_name("1_backlog"), "Backlog");
-        assert_eq!(stage_display_name("2_current"), "Current");
-        assert_eq!(stage_display_name("3_qa"), "QA");
-        assert_eq!(stage_display_name("4_merge"), "Merge");
-        assert_eq!(stage_display_name("5_done"), "Done");
-        assert_eq!(stage_display_name("6_archived"), "Archived");
+        assert_eq!(stage_display_name("backlog"), "Backlog");
+        assert_eq!(stage_display_name("coding"), "Current");
+        assert_eq!(stage_display_name("qa"), "QA");
+        assert_eq!(stage_display_name("merge"), "Merge");
+        assert_eq!(stage_display_name("done"), "Done");
+        assert_eq!(stage_display_name("archived"), "Archived");
         assert_eq!(stage_display_name("unknown"), "Unknown");
     }