feat: agent detection + selective install (v2.4.0)

- post-install.sh: reads openclaw.json `list` array via python3,
  presents a numbered menu (all/specific/quit), installs SKILL.md
  to <workspace>/skills/memory-continuity/SKILL.md for chosen agents;
  falls back to manual path prompt if config absent
- post-install.sh: remove gateway restart step
- verify.sh: add --all-agents flag that runs the 3-layer check
  (SKILL.md, continuity_doctor.py, CURRENT_STATE.md) across every
  detected agent workspace and reports pass/fail per agent
- openclaw.plugin.json: bump version to 2.4.0

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

openclaw.plugin.json

@@ -2,7 +2,7 @@
   "id": "memory-continuity",
   "name": "Memory Continuity",
   "description": "Preserves working state across /new, reset, compaction, and gateway restarts via a simple markdown checkpoint file.",
-  "version": "2.3.0",
+  "version": "2.4.0",
   "source": "https://github.com/dtzp555-max/memory-continuity",
   "configSchema": {
     "type": "object",