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

adapters/pi/adapter.sh

@@ -18,11 +18,16 @@ if [ -n "${MOSAIC_WORKSPACE:-}" ]; then
     cd "$MOSAIC_WORKSPACE"
 fi
 
-# Session (M6): persistent named session directory; resume the most recent
-# session in that directory when one exists (pi documented flags).
-# Default remains ephemeral (--no-session) when no session is declared.
+# Session (M6/M11): default ephemeral (--no-session). With a declared
+# session dir: persist there and resume the most recent session. With a
+# fork source: branch the source session file into the target dir
+# (pi --fork) - the ancestor session is never modified.
 SESSION_FLAGS="--no-session"
-if [ -n "${MOSAIC_SESSION_DIR:-}" ]; then
+if [ -n "${MOSAIC_SESSION_FORK:-}" ]; then
+    [ -n "${MOSAIC_SESSION_DIR:-}" ] || { echo "pi adapter: session fork requires MOSAIC_SESSION_DIR" >&2; exit 2; }
+    mkdir -p "$MOSAIC_SESSION_DIR"
+    SESSION_FLAGS="--fork $MOSAIC_SESSION_FORK --session-dir $MOSAIC_SESSION_DIR"
+elif [ -n "${MOSAIC_SESSION_DIR:-}" ]; then
     mkdir -p "$MOSAIC_SESSION_DIR"
     SESSION_FLAGS="--session-dir $MOSAIC_SESSION_DIR"
     if [ -n "$(ls -A "$MOSAIC_SESSION_DIR" 2>/dev/null)" ]; then