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

compose.yaml

@@ -18,6 +18,8 @@ services:
       # Workspace + capabilities (set by the task runner; M5)
       MOSAIC_WORKSPACE: ${MOSAIC_WORKSPACE:-}
       MOSAIC_TOOLS: ${MOSAIC_TOOLS:-}
+      # Persistent named session dir (set by the task runner; M6)
+      MOSAIC_SESSION_DIR: ${MOSAIC_SESSION_DIR:-}
       # mock adapter only: verbatim response for deterministic seam tests
       MOSAIC_MOCK_RESPONSE: ${MOSAIC_MOCK_RESPONSE:-}
       # Documented container auth alternative: provider API key via