feat(x/core): session layer with write-through index (stage 3)

SessionsImpl per session-design.md, constructor-injected and tested
entirely against a mocked ITurnRuntime:

- sendMessage: per-session lock, typed TurnNotSettledError while the
  latest turn is non-terminal (terminal statuses including failed/
  cancelled allow continuation), context as { previousTurnId } ref,
  turn-file-first write ordering (a failed session append leaves a
  benign orphan turn and no advance), denormalized agent/model on
  turn_appended, default title from the first message.
- Dedicated respondToAskHuman endpoint (async_tool_result wrapper);
  respondToPermission / deliverAsyncToolResult pass turn-runtime
  rejections through. stopTurn aborts a live advance or cancels an
  at-rest turn; resumeTurn re-enters idle turns (never at startup).
- In-memory index: startup scan (session files + each latest turn for
  status; corrupt files yield errored entries without aborting),
  write-through updates, deletion guard against late-settle
  resurrection. Bus events (turn-event / index-changed) defined in
  @x/shared as the renderer IPC contract.
- FSSessionRepo: date-partitioned append-only JSONL + list/delete;
  deleteSession removes the session file only.
- runHeadlessTurn: standalone turns (sessionId null, auto permission,
  no human) for background/knowledge/scheduled callers.

29 new tests covering the session-design §13 matrix.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/x/packages/shared/src/sessions.ts

@@ -1,5 +1,9 @@
 import { z } from "zod";
-import { ModelDescriptor, type TurnStatus } from "./turns.js";
+import {
+    ModelDescriptor,
+    type TurnStatus,
+    type TurnStreamEvent,
+} from "./turns.js";
 
 // Durable session contract for the session layer (see
 // packages/core/docs/session-design.md). A session is an append-only chain
@@ -167,6 +171,22 @@ export interface SessionIndexEntry {
     error?: string;
 }
 
+// What the renderer's single feed consumer receives over IPC: live turn
+// stream events (durable events + ephemeral deltas) and index updates.
+// entry: null signals deletion.
+export type SessionBusEvent =
+    | {
+          kind: "turn-event";
+          sessionId: string;
+          turnId: string;
+          event: TurnStreamEvent;
+      }
+    | {
+          kind: "index-changed";
+          sessionId: string;
+          entry: SessionIndexEntry | null;
+      };
+
 export function sessionIndexEntry(
     state: SessionState,
     latestTurnStatus: SessionLatestTurnStatus,