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

adapters/pi/adapter.sh

@@ -18,6 +18,18 @@ 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_FLAGS="--no-session"
+if [ -n "${MOSAIC_SESSION_DIR:-}" ]; then
+    mkdir -p "$MOSAIC_SESSION_DIR"
+    SESSION_FLAGS="--session-dir $MOSAIC_SESSION_DIR"
+    if [ -n "$(ls -A "$MOSAIC_SESSION_DIR" 2>/dev/null)" ]; then
+        SESSION_FLAGS="$SESSION_FLAGS -c"
+    fi
+fi
+
 # Capabilities (M5): explicit allowlist or no tools.
 TOOLS_FLAG="--no-tools"
 [ -n "${MOSAIC_TOOLS:-}" ] && TOOLS_FLAG="--tools $MOSAIC_TOOLS"
@@ -30,13 +42,13 @@ TOOLS_FLAG="--no-tools"
 #   --offline      no startup network operations (update checks/telemetry)
 exec pi \
     --offline \
-    --no-session \
     --no-extensions \
     --no-skills \
     --no-prompt-templates \
     --no-themes \
     --no-context-files \
     $TOOLS_FLAG \
+    $SESSION_FLAGS \
     --provider "$PI_PROVIDER" \
     --model "$PI_MODEL" \
     --system-prompt "$(cat "$MOSAIC_SYSTEM_PROMPT_FILE")" \