feat(sessions): named persistent sessions with resume (L1) (#22, #23)

- task schema: optional session (named id) -> persistent session dir at
  dataRoot/sessions/<name>, isolated per name
- pi adapter: --session-dir when declared (ephemeral --no-session stays
  the default otherwise); -c resumes the most recent session when present
- compose passthrough; result.json records session
- fixtures: tasks/session-demo-1.json (teach) + session-demo-2.json (recall)
- E2E: teach -> REMEMBERED + host-side session JSONL; resume -> recalled
  'mosaico' exactly; single continued session file

Closes #22, closes #23

tasks/session-demo-1.json

@@ -0,0 +1,8 @@
+{
+  "taskVersion": 1,
+  "id": "t-session-teach",
+  "prompt": "Remember this code word for later: mosaico. Reply with exactly: REMEMBERED",
+  "session": "demo",
+  "expectExact": "REMEMBERED",
+  "timeoutSeconds": 180
+}

tasks/session-demo-2.json

@@ -0,0 +1,7 @@
+{
+  "taskVersion": 1,
+  "id": "t-session-recall",
+  "prompt": "What code word did I ask you to remember earlier in this session? Reply with only the code word.",
+  "session": "demo",
+  "timeoutSeconds": 180
+}