feat(sessions): sessionForkFrom - branch conversations from a common ancestor (#33)

- task schema: optional sessionForkFrom (source session name); requires
  session target; self-fork rejected
- runner: resolves source newest .jsonl (fail 4 if none/outside dataRoot);
  passes MOSAIC_SESSION_FORK + MOSAIC_SESSION_DIR; result records lineage
- pi adapter: --fork <source> --session-dir <target> when forking;
  ephemeral default unchanged; plain session resume unchanged
- compose passthrough; RELEASE -> 0.0.7 (adapter changed)
- suite +9 cases (58 total): plumbing via mock stderr, validation
  negatives, live fork - child recalls ancestor code word, ancestor
  session file untouched

Closes #33

compose.yaml

@@ -18,8 +18,9 @@ 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)
+      # Persistent named session dir + optional fork source (M6/M11)
       MOSAIC_SESSION_DIR: ${MOSAIC_SESSION_DIR:-}
+      MOSAIC_SESSION_FORK: ${MOSAIC_SESSION_FORK:-}
       # mock adapter only: verbatim response for deterministic seam tests
       MOSAIC_MOCK_RESPONSE: ${MOSAIC_MOCK_RESPONSE:-}
       # Documented container auth alternative: provider API key via