Handoff-style /model switch command and per-turn thinking level (#64)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/server/src/services/session-service.ts

@@ -16,6 +16,7 @@ import path from "node:path";
 import { open, readdir } from "node:fs/promises";
 import {
   createAgent,
+  findLatestTraceFile,
   isSessionMeta,
   parseTraceLines,
   readTraceTolerant,
@@ -389,6 +390,21 @@ export class SessionService {
     return this.toInfo(row, false);
   }
 
+  /**
+   * Absolute path of a Session's **latest** Trace file (the current context shard);
+   * undefined when no Trace exists. Costs a directory walk, so only the single-session
+   * GET includes it in the DTO (see SessionInfo.tracePath) — the web's `/model` switch
+   * hands it to the new session's `<model_switch_from>` block so the model can read the
+   * source history itself when it needs it.
+   */
+  async latestTracePath(row: SessionRow): Promise<string | undefined> {
+    const located = await findLatestTraceFile(
+      tracesDir(this.deps.root, row.projectId, row.agentId),
+      row.sessionId,
+    );
+    return located?.path;
+  }
+
   /**
    * One walk over the Trace directory: session_id → its **earliest** shard (the shard
    * whose head carries the original session_meta). Discovery (which Sessions have