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/workflow/pipeline.rs

@@ -137,6 +137,12 @@ pub fn load_pipeline_state(ctx: &AppContext) -> Result<PipelineState, String> {
             },
             epic_id,
         };
+        // Frozen items (CRDT flag) are routed to the backlog bucket regardless
+        // of their underlying stage — they're paused, not progressing.
+        if item.is_frozen() {
+            state.backlog.push(story);
+            continue;
+        }
         match &item.stage {
             Stage::Upcoming => state.backlog.push(story), // upcoming shown with backlog
             Stage::Backlog => state.backlog.push(story),
@@ -147,7 +153,6 @@ pub fn load_pipeline_state(ctx: &AppContext) -> Result<PipelineState, String> {
             Stage::MergeFailure { .. } => state.merge.push(story), // show merge failures with merge
             Stage::Done { .. } => state.done.push(story),
             Stage::Archived { .. } => {} // skip archived
-            Stage::Frozen { .. } => state.backlog.push(story), // show frozen with backlog
         }
     }