feat(plugins): add slash command registration for plugins (#2359)

Plugins can now register slash commands via ctx.register_command()
in their register() function. Commands automatically appear in:
- /help and COMMANDS_BY_CATEGORY (under 'Plugins' category)
- Tab autocomplete in CLI
- Telegram bot menu
- Slack subcommand mapping
- Gateway dispatch

Handler signature: handler(args: str) -> str | None
Async handlers are supported in gateway context.

Changes:
- commands.py: add register_plugin_command() and rebuild_lookups()
- plugins.py: add register_command() to PluginContext, track in
  PluginManager._plugin_commands and LoadedPlugin.commands_registered
- cli.py: dispatch plugin commands in process_command()
- gateway/run.py: dispatch plugin commands before skill commands
- tests: 5 new tests for registration, help, tracking, handler, gateway
- docs: update plugins feature page and build guide

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

@@ -4,7 +4,7 @@ sidebar_position: 20
 
 # Plugins
 
-Hermes has a plugin system for adding custom tools, hooks, and integrations without modifying core code.
+Hermes has a plugin system for adding custom tools, hooks, slash commands, and integrations without modifying core code.
 
 **→ [Build a Hermes Plugin](/docs/guides/build-a-hermes-plugin)** — step-by-step guide with a complete working example.
 
@@ -30,6 +30,7 @@ Project-local plugins under `./.hermes/plugins/` are disabled by default. Enable
 |-----------|-----|
 | Add tools | `ctx.register_tool(name, schema, handler)` |
 | Add hooks | `ctx.register_hook("post_tool_call", callback)` |
+| Add slash commands | `ctx.register_command("mycommand", handler)` |
 | Ship data files | `Path(__file__).parent / "data" / "file.yaml"` |
 | Bundle skills | Copy `skill.md` to `~/.hermes/skills/` at load time |
 | Gate on env vars | `requires_env: [API_KEY]` in plugin.yaml |
@@ -54,6 +55,33 @@ Project-local plugins under `./.hermes/plugins/` are disabled by default. Enable
 | `on_session_start` | Session begins |
 | `on_session_end` | Session ends |
 
+## Slash commands
+
+Plugins can register slash commands that work in both CLI and messaging platforms:
+
+```python
+def register(ctx):
+    ctx.register_command(
+        name="greet",
+        handler=lambda args: f"Hello, {args or 'world'}!",
+        description="Greet someone",
+        args_hint="[name]",
+        aliases=("hi",),
+    )
+```
+
+The handler receives the argument string (everything after `/greet`) and returns a string to display. Registered commands automatically appear in `/help`, tab autocomplete, Telegram bot menu, and Slack subcommand mapping.
+
+| Parameter | Description |
+|-----------|-------------|
+| `name` | Command name without slash |
+| `handler` | Callable that takes `args: str` and returns `str | None` |
+| `description` | Shown in `/help` |
+| `args_hint` | Usage hint, e.g. `"[name]"` |
+| `aliases` | Tuple of alternative names |
+| `cli_only` | Only available in CLI |
+| `gateway_only` | Only available in messaging platforms |
+
 ## Managing plugins
 
 ```