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/pipeline_state/transition.rs

@@ -59,10 +59,6 @@ pub enum PipelineEvent {
     Close,
     /// Manual demotion back to backlog from an active stage.
     Demote,
-    /// Freeze the story at its current stage (suspends pipeline and auto-assign).
-    Freeze,
-    /// Unfreeze the story, restoring it to the stage it was at when frozen.
-    Unfreeze,
 }
 
 // ── Per-node execution events ───────────────────────────────────────────────
@@ -102,8 +98,6 @@ pub fn event_label(e: &PipelineEvent) -> &'static str {
         PipelineEvent::Triage => "Triage",
         PipelineEvent::Close => "Close",
         PipelineEvent::Demote => "Demote",
-        PipelineEvent::Freeze => "Freeze",
-        PipelineEvent::Unfreeze => "Unfreeze",
     }
 }
 
@@ -267,17 +261,6 @@ pub fn transition(state: Stage, event: PipelineEvent) -> Result<Stage, Transitio
             Unblock,
         ) => Ok(Backlog),
 
-        // ── Freeze: any active stage → Frozen(resume_to=current) ────────
-        (stage @ (Upcoming | Backlog | Coding | Qa), Freeze) => Ok(Frozen {
-            resume_to: Box::new(stage),
-        }),
-        (stage @ Merge { .. }, Freeze) => Ok(Frozen {
-            resume_to: Box::new(stage),
-        }),
-
-        // ── Unfreeze: Frozen → resume_to ─────────────────────────────────
-        (Frozen { resume_to }, Unfreeze) => Ok(*resume_to),
-
         // ── Everything else is invalid ──────────────────────────────────
         _ => Err(invalid()),
     }