feat(plugins): make all plugins opt-in by default

Plugins now require explicit consent to load. Discovery still finds every
plugin — user-installed, bundled, and pip — so they all show up in
`hermes plugins` and `/plugins`, but the loader only instantiates
plugins whose name appears in `plugins.enabled` in config.yaml. This
removes the previous ambient-execution risk where a newly-installed or
bundled plugin could register hooks, tools, and commands on first run
without the user opting in.

The three-state model is now explicit:
  enabled     — in plugins.enabled, loads on next session
  disabled    — in plugins.disabled, never loads (wins over enabled)
  not enabled — discovered but never opted in (default for new installs)

`hermes plugins install <repo>` prompts "Enable 'name' now? [y/N]"
(defaults to no). New `--enable` / `--no-enable` flags skip the prompt
for scripted installs. `hermes plugins enable/disable` manage both lists
so a disabled plugin stays explicitly off even if something later adds
it to enabled.

Config migration (schema v20 → v21): existing user plugins already
installed under ~/.hermes/plugins/ (minus anything in plugins.disabled)
are auto-grandfathered into plugins.enabled so upgrades don't silently
break working setups. Bundled plugins are NOT grandfathered — even
existing users have to opt in explicitly.

Also: HERMES_DISABLE_BUNDLED_PLUGINS env var removed (redundant with
opt-in default), cmd_list now shows bundled + user plugins together with
their three-state status, interactive UI tags bundled entries
[bundled], docs updated across plugins.md and built-in-plugins.md.

Validation: 442 plugin/config tests pass. E2E: fresh install discovers
disk-cleanup but does not load it; `hermes plugins enable disk-cleanup`
activates hooks; migration grandfathers existing user plugins correctly
while leaving bundled plugins off.

website/docs/user-guide/features/built-in-plugins.md

@@ -24,23 +24,31 @@ On name collision, later sources win — a user plugin named `disk-cleanup` woul
 
 `plugins/memory/` and `plugins/context_engine/` are deliberately excluded from bundled scanning. Those directories use their own discovery paths because memory providers and context engines are single-select providers configured through `hermes memory setup` / `context.engine` in config.
 
-Bundled plugins respect the same disable mechanism as any other plugin:
+## Bundled plugins are opt-in
+
+Bundled plugins ship disabled. Discovery finds them (they appear in `hermes plugins list` and the interactive `hermes plugins` UI), but none load until you explicitly enable them:
+
+```bash
+hermes plugins enable disk-cleanup
+```
+
+Or via `~/.hermes/config.yaml`:
 
 ```yaml
-# ~/.hermes/config.yaml
 plugins:
-  disabled:
+  enabled:
     - disk-cleanup
 ```
 
-Or suppress every bundled plugin at once with an env var:
+This is the same mechanism user-installed plugins use. Bundled plugins are never auto-enabled — not on fresh install, not for existing users upgrading to a newer Hermes. You always opt in explicitly.
+
+To turn a bundled plugin off again:
 
 ```bash
-HERMES_DISABLE_BUNDLED_PLUGINS=1 hermes chat
+hermes plugins disable disk-cleanup
+# or: remove it from plugins.enabled in config.yaml
 ```
 
-The test suite sets `HERMES_DISABLE_BUNDLED_PLUGINS=1` in its hermetic fixture — tests that exercise bundled discovery clear it explicitly.
-
 ## Currently shipped
 
 ### disk-cleanup
@@ -87,14 +95,9 @@ Auto-tracks and removes ephemeral files created during sessions — test scripts
 
 **Safety** — cleanup only ever touches paths under `HERMES_HOME` or `/tmp/hermes-*`. Windows mounts (`/mnt/c/...`) are rejected. Well-known top-level state dirs (`logs/`, `memories/`, `sessions/`, `cron/`, `cache/`, `skills/`, `plugins/`, `disk-cleanup/` itself) are never removed even when empty — a fresh install does not get gutted on first session end.
 
-To turn it off without uninstalling:
+**Enabling:** `hermes plugins enable disk-cleanup` (or check the box in `hermes plugins`).
 
-```yaml
-# ~/.hermes/config.yaml
-plugins:
-  disabled:
-    - disk-cleanup
-```
+**Disabling again:** `hermes plugins disable disk-cleanup`.
 
 ## Adding a bundled plugin