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/chat/lookup.rs

@@ -33,18 +33,14 @@ pub(crate) fn find_story_by_number(
     if let Some(items) = crate::crdt_state::read_all_items() {
         for item in items {
             if item.story_id().split('_').next().unwrap_or("") == number {
+                let stage_dir = item.stage().as_dir().to_string();
                 let path = project_root
                     .join(".huskies")
                     .join("work")
-                    .join(item.stage_str())
+                    .join(&stage_dir)
                     .join(format!("{}.md", item.story_id()));
                 let content = crate::db::read_content(item.story_id());
-                return Some((
-                    item.story_id().to_string(),
-                    item.stage_str().to_string(),
-                    path,
-                    content,
-                ));
+                return Some((item.story_id().to_string(), stage_dir, path, content));
             }
         }
     }
@@ -58,8 +54,8 @@ pub(crate) fn find_story_by_number(
             continue;
         }
         let stage_dir = crate::crdt_state::read_item(&id)
-            .map(|v| v.stage_str().to_string())
-            .unwrap_or_else(|| "1_backlog".to_string());
+            .map(|v| v.stage().as_dir().to_string())
+            .unwrap_or_else(|| "backlog".to_string());
         let path = project_root
             .join(".huskies")
             .join("work")