feat(plugins): convert disk-guardian skill into a bundled plugin

Rewires @LVT382009's disk-guardian (PR #12212) from a skill-plus-script
into a plugin that runs entirely via hooks — no agent compliance needed.

- post_tool_call hook auto-tracks files created by write_file / terminal
  / patch when they match test_/tmp_/*.test.* patterns under HERMES_HOME
- on_session_end hook runs cmd_quick cleanup when test files were
  auto-tracked during the turn; stays quiet otherwise
- /disk-guardian slash command keeps status / dry-run / quick / deep /
  track / forget for manual use
- Deterministic cleanup rules, path safety, atomic writes, and audit
  logging preserved from the original contribution
- Protect well-known top-level state dirs (logs/, memories/, sessions/,
  cron/, cache/, etc.) from empty-dir removal so fresh installs don't
  get gutted on first session end

The plugin system gains a bundled-plugin discovery path (<repo>/plugins/
<name>/) alongside user/project/entry-point sources. Memory and
context_engine subdirs are skipped — they keep their own discovery
paths. HERMES_DISABLE_BUNDLED_PLUGINS=1 suppresses the scan; the test
conftest sets it by default so existing plugin tests stay clean.

Co-authored-by: LVT382009 <levantam.98.2324@gmail.com>

plugins/disk-guardian/plugin.yaml

@@ -0,0 +1,7 @@
+name: disk-guardian
+version: 2.0.0
+description: "Auto-track and clean up ephemeral files (test scripts, temp outputs, cron logs) created during Hermes sessions. Runs via plugin hooks — no agent action required."
+author: "@LVT382009 (original), NousResearch (plugin port)"
+hooks:
+  - post_tool_call
+  - on_session_end