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/README.md

@@ -0,0 +1,51 @@
+# disk-guardian
+
+Auto-tracks and cleans up ephemeral files created during Hermes Agent
+sessions — test scripts, temp outputs, cron logs, stale chrome profiles.
+Scoped strictly to `$HERMES_HOME` and `/tmp/hermes-*`.
+
+Originally contributed by [@LVT382009](https://github.com/LVT382009) as a
+skill in PR #12212.  Ported to the plugin system so the behaviour runs
+automatically via `post_tool_call` and `on_session_end` hooks — the agent
+never needs to remember to call a tool.
+
+## How it works
+
+| Hook | Behaviour |
+|---|---|
+| `post_tool_call` | When `write_file` / `terminal` / `patch` creates a file matching `test_*`, `tmp_*`, or `*.test.*` inside `HERMES_HOME`, track it silently as `test` / `temp` / `cron-output`. |
+| `on_session_end` | If any test files were auto-tracked during this turn, run `quick` cleanup (no prompts). |
+
+Deletion rules (same as the original PR):
+
+| Category | Threshold | Confirmation |
+|---|---|---|
+| `test` | every session end | Never |
+| `temp` | >7 days since tracked | Never |
+| `cron-output` | >14 days since tracked | Never |
+| empty dirs under HERMES_HOME | always | Never |
+| `research` | >30 days, beyond 10 newest | Always (deep only) |
+| `chrome-profile` | >14 days since tracked | Always (deep only) |
+| files >500 MB | never auto | Always (deep only) |
+
+## Slash command
+
+```
+/disk-guardian status                     # breakdown + top-10 largest
+/disk-guardian dry-run                    # preview without deleting
+/disk-guardian quick                      # run safe cleanup now
+/disk-guardian deep                       # quick + list items needing prompt
+/disk-guardian track <path> <category>    # manual tracking
+/disk-guardian forget <path>              # stop tracking
+```
+
+## Safety
+
+- `is_safe_path()` rejects anything outside `HERMES_HOME` or `/tmp/hermes-*`
+- Windows mounts (`/mnt/c` etc.) are rejected
+- The state directory `$HERMES_HOME/disk-guardian/` is itself excluded
+- `$HERMES_HOME/logs/`, `memories/`, `sessions/`, `skills/`, `plugins/`,
+  and config files are never tracked
+- Backup/restore is scoped to `tracked.json` — the plugin never touches
+  agent logs
+- Atomic writes: `.tmp` → backup → rename