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

hermes_cli/commands.py

@@ -137,7 +137,7 @@ COMMAND_REGISTRY: list[CommandDef] = [
 
 
 # ---------------------------------------------------------------------------
-# Derived lookups -- rebuilt once at import time
+# Derived lookups -- rebuilt once at import time, refreshed by rebuild_lookups()
 # ---------------------------------------------------------------------------
 
 def _build_command_lookup() -> dict[str, CommandDef]:
@@ -161,6 +161,58 @@ def resolve_command(name: str) -> CommandDef | None:
     return _COMMAND_LOOKUP.get(name.lower().lstrip("/"))
 
 
+def register_plugin_command(cmd: CommandDef) -> None:
+    """Append a plugin-defined command to the registry and refresh lookups."""
+    COMMAND_REGISTRY.append(cmd)
+    rebuild_lookups()
+
+
+def rebuild_lookups() -> None:
+    """Rebuild all derived lookup dicts from the current COMMAND_REGISTRY.
+
+    Called after plugin commands are registered so they appear in help,
+    autocomplete, gateway dispatch, Telegram menu, and Slack mapping.
+    """
+    global GATEWAY_KNOWN_COMMANDS
+
+    _COMMAND_LOOKUP.clear()
+    _COMMAND_LOOKUP.update(_build_command_lookup())
+
+    COMMANDS.clear()
+    for cmd in COMMAND_REGISTRY:
+        if not cmd.gateway_only:
+            COMMANDS[f"/{cmd.name}"] = _build_description(cmd)
+            for alias in cmd.aliases:
+                COMMANDS[f"/{alias}"] = f"{cmd.description} (alias for /{cmd.name})"
+
+    COMMANDS_BY_CATEGORY.clear()
+    for cmd in COMMAND_REGISTRY:
+        if not cmd.gateway_only:
+            cat = COMMANDS_BY_CATEGORY.setdefault(cmd.category, {})
+            cat[f"/{cmd.name}"] = COMMANDS[f"/{cmd.name}"]
+            for alias in cmd.aliases:
+                cat[f"/{alias}"] = COMMANDS[f"/{alias}"]
+
+    SUBCOMMANDS.clear()
+    for cmd in COMMAND_REGISTRY:
+        if cmd.subcommands:
+            SUBCOMMANDS[f"/{cmd.name}"] = list(cmd.subcommands)
+    for cmd in COMMAND_REGISTRY:
+        key = f"/{cmd.name}"
+        if key in SUBCOMMANDS or not cmd.args_hint:
+            continue
+        m = _PIPE_SUBS_RE.search(cmd.args_hint)
+        if m:
+            SUBCOMMANDS[key] = m.group(0).split("|")
+
+    GATEWAY_KNOWN_COMMANDS = frozenset(
+        name
+        for cmd in COMMAND_REGISTRY
+        if not cmd.cli_only
+        for name in (cmd.name, *cmd.aliases)
+    )
+
+
 def _build_description(cmd: CommandDef) -> str:
     """Build a CLI-facing description string including usage hint."""
     if cmd.args_hint: