docs(plugins): rename disk-guardian to disk-cleanup + bundled-plugins docs

The original name was cute but non-obvious; disk-cleanup says what it
does. Plugin directory, script, state path, log lines, slash command,
and test module all renamed. No user-visible state exists yet, so no
migration path is needed.

New website page "Built-in Plugins" documents the <repo>/plugins/<name>/
source, how discovery interacts with user/project plugins, the
HERMES_DISABLE_BUNDLED_PLUGINS escape hatch, disk-cleanup's hook
behaviour and deletion rules, and guidance on when a plugin belongs
bundled vs. user-installable. Added to the Features → Core sidebar next
to the main Plugins page, with a cross-reference from plugins.md.

plugins/disk-cleanup/README.md

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