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/status_tools.rs

@@ -158,16 +158,16 @@ pub(super) async fn tool_status(args: &Value, ctx: &AppContext) -> Result<String
 
     let root = ctx.state.get_project_root()?;
 
-    // Read from CRDT/DB content store — verify the item is in 2_current.
+    // Read from CRDT/DB content store — verify the item is in coding.
     let typed_item = crate::pipeline_state::read_typed(story_id)
         .map_err(|e| format!("Failed to read pipeline state: {e}"))?
         .ok_or_else(|| format!(
-            "Story '{story_id}' not found in work/2_current/. Check the story_id and ensure it is in the current stage."
+            "Story '{story_id}' not found in coding stage. Check the story_id and ensure it is in the current stage."
         ))?;
 
-    if typed_item.stage.dir_name() != "2_current" {
+    if !matches!(typed_item.stage, crate::pipeline_state::Stage::Coding) {
         return Err(format!(
-            "Story '{story_id}' not found in work/2_current/. Check the story_id and ensure it is in the current stage."
+            "Story '{story_id}' not found in coding stage. Check the story_id and ensure it is in the current stage."
         ));
     }
 
@@ -353,7 +353,7 @@ mod tests {
         let ctx = crate::http::context::AppContext::new_test(tmp.path().to_path_buf());
         let result = tool_status(&json!({"story_id": "999_story_nonexistent"}), &ctx).await;
         assert!(result.is_err());
-        assert!(result.unwrap_err().contains("not found in work/2_current/"));
+        assert!(result.unwrap_err().contains("not found in coding stage"));
     }
 
     #[tokio::test]