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/transport/matrix/assign.rs

@@ -313,7 +313,7 @@ mod tests {
             None,
         );
         // Seed CRDT so set_agent can write to the item.
-        crate::crdt_state::write_item(
+        crate::crdt_state::write_item_str(
             "9972_story_test",
             "1_backlog",
             Some("Test Feature"),
@@ -369,7 +369,7 @@ mod tests {
             "---\nname: Small Story\n---\n",
             None,
         );
-        crate::crdt_state::write_item(
+        crate::crdt_state::write_item_str(
             "9973_story_small",
             "1_backlog",
             Some("Small Story"),
@@ -420,7 +420,7 @@ mod tests {
             "---\nname: Existing\nagent: coder-sonnet\n---\n",
             None,
         );
-        crate::crdt_state::write_item(
+        crate::crdt_state::write_item_str(
             "9974_story_existing",
             "1_backlog",
             Some("Existing"),

server/src/chat/transport/matrix/delete.rs

@@ -112,7 +112,6 @@ fn stage_display_name(stage: &str) -> &str {
         Some(Stage::Done { .. }) => "done",
         Some(Stage::Archived { .. }) => "archived",
         Some(Stage::MergeFailure { .. }) => "merge-failure",
-        Some(Stage::Frozen { .. }) => "frozen",
         None => stage,
     }
 }
@@ -239,7 +238,7 @@ mod tests {
         let story_number = "9977";
 
         // Seed in CRDT.
-        crate::crdt_state::write_item(
+        crate::crdt_state::write_item_str(
             story_id,
             "1_backlog",
             Some("CRDT Tombstone Check"),