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/agents/pool/auto_assign/scan.rs

@@ -22,9 +22,29 @@ pub(super) fn scan_stage_items(_project_root: &Path, stage_dir: &str) -> Vec<Str
     use std::collections::BTreeSet;
     let mut items = BTreeSet::new();
 
+    // Accept legacy directory-style strings (`"2_current"`, `"4_merge"`,
+    // etc.) at the boundary; `Stage::from_dir` itself is strict post-934
+    // stage 6, so we normalise here.
+    let normalised = match stage_dir {
+        "0_upcoming" => "upcoming",
+        "1_backlog" => "backlog",
+        "2_current" => "coding",
+        "2_blocked" => "blocked",
+        "3_qa" => "qa",
+        "4_merge" => "merge",
+        "4_merge_failure" => "merge_failure",
+        "5_done" => "done",
+        "6_archived" => "archived",
+        other => other,
+    };
+    let Some(want) = crate::pipeline_state::Stage::from_dir(normalised) else {
+        return Vec::new();
+    };
+    let want = want.dir_name();
+
     // CRDT is the only source of truth — no filesystem fallback.
     for item in crate::pipeline_state::read_all_typed() {
-        if item.stage.dir_name() == stage_dir {
+        if item.stage.dir_name() == want {
             items.insert(item.story_id.0.clone());
         }
     }