feat: implement register_command() on plugin context (#10626)

Complete the half-built plugin slash command system. The dispatch
code in cli.py and gateway/run.py already called
get_plugin_command_handler() but the registration side was never
implemented.

Changes:
- Add register_command() to PluginContext — stores handler,
  description, and plugin name; normalizes names; rejects conflicts
  with built-in commands
- Add _plugin_commands dict to PluginManager
- Add commands_registered tracking on LoadedPlugin
- Add get_plugin_command_handler() and get_plugin_commands()
  module-level convenience functions
- Fix commands.py to use actual plugin description in Telegram
  bot menu (was hardcoded 'Plugin command')
- Add plugin commands to SlashCommandCompleter autocomplete
- Show command count in /plugins display
- 12 new tests covering registration, conflict detection,
  normalization, handler dispatch, and introspection

Closes #10495

tests/hermes_cli/test_plugins.py

@@ -18,6 +18,8 @@ from hermes_cli.plugins import (
     PluginManager,
     PluginManifest,
     get_plugin_manager,
+    get_plugin_command_handler,
+    get_plugin_commands,
     get_pre_tool_call_block_message,
     discover_plugins,
     invoke_hook,
@@ -605,7 +607,160 @@ class TestPreLlmCallTargetRouting:
         assert "plain text C" in _plugin_user_context
 
 
-# NOTE: TestPluginCommands removed – register_command() was never implemented
-# in PluginContext (hermes_cli/plugins.py).  The tests referenced _plugin_commands,
-# commands_registered, get_plugin_command_handler, and GATEWAY_KNOWN_COMMANDS
-# integration — all of which are unimplemented features.
+# ── TestPluginCommands ────────────────────────────────────────────────────
+
+
+class TestPluginCommands:
+    """Tests for plugin slash command registration via register_command()."""
+
+    def test_register_command_basic(self):
+        """register_command() stores handler, description, and plugin name."""
+        mgr = PluginManager()
+        manifest = PluginManifest(name="test-plugin", source="user")
+        ctx = PluginContext(manifest, mgr)
+
+        handler = lambda args: f"echo {args}"
+        ctx.register_command("mycmd", handler, description="My custom command")
+
+        assert "mycmd" in mgr._plugin_commands
+        entry = mgr._plugin_commands["mycmd"]
+        assert entry["handler"] is handler
+        assert entry["description"] == "My custom command"
+        assert entry["plugin"] == "test-plugin"
+
+    def test_register_command_normalizes_name(self):
+        """Names are lowercased, stripped, and leading slashes removed."""
+        mgr = PluginManager()
+        manifest = PluginManifest(name="test-plugin", source="user")
+        ctx = PluginContext(manifest, mgr)
+
+        ctx.register_command("/MyCmd ", lambda a: a, description="test")
+        assert "mycmd" in mgr._plugin_commands
+        assert "/MyCmd " not in mgr._plugin_commands
+
+    def test_register_command_empty_name_rejected(self, caplog):
+        """Empty name after normalization is rejected with a warning."""
+        mgr = PluginManager()
+        manifest = PluginManifest(name="test-plugin", source="user")
+        ctx = PluginContext(manifest, mgr)
+
+        with caplog.at_level(logging.WARNING):
+            ctx.register_command("", lambda a: a)
+        assert len(mgr._plugin_commands) == 0
+        assert "empty name" in caplog.text
+
+    def test_register_command_builtin_conflict_rejected(self, caplog):
+        """Commands that conflict with built-in names are rejected."""
+        mgr = PluginManager()
+        manifest = PluginManifest(name="test-plugin", source="user")
+        ctx = PluginContext(manifest, mgr)
+
+        with caplog.at_level(logging.WARNING):
+            ctx.register_command("help", lambda a: a)
+        assert "help" not in mgr._plugin_commands
+        assert "conflicts" in caplog.text.lower()
+
+    def test_register_command_default_description(self):
+        """Missing description defaults to 'Plugin command'."""
+        mgr = PluginManager()
+        manifest = PluginManifest(name="test-plugin", source="user")
+        ctx = PluginContext(manifest, mgr)
+
+        ctx.register_command("status-cmd", lambda a: a)
+        assert mgr._plugin_commands["status-cmd"]["description"] == "Plugin command"
+
+    def test_get_plugin_command_handler_found(self):
+        """get_plugin_command_handler() returns the handler for a registered command."""
+        mgr = PluginManager()
+        manifest = PluginManifest(name="test-plugin", source="user")
+        ctx = PluginContext(manifest, mgr)
+
+        handler = lambda args: f"result: {args}"
+        ctx.register_command("mycmd", handler, description="test")
+
+        with patch("hermes_cli.plugins._plugin_manager", mgr):
+            result = get_plugin_command_handler("mycmd")
+            assert result is handler
+
+    def test_get_plugin_command_handler_not_found(self):
+        """get_plugin_command_handler() returns None for unregistered commands."""
+        mgr = PluginManager()
+        with patch("hermes_cli.plugins._plugin_manager", mgr):
+            assert get_plugin_command_handler("nonexistent") is None
+
+    def test_get_plugin_commands_returns_dict(self):
+        """get_plugin_commands() returns the full commands dict."""
+        mgr = PluginManager()
+        manifest = PluginManifest(name="test-plugin", source="user")
+        ctx = PluginContext(manifest, mgr)
+        ctx.register_command("cmd-a", lambda a: a, description="A")
+        ctx.register_command("cmd-b", lambda a: a, description="B")
+
+        with patch("hermes_cli.plugins._plugin_manager", mgr):
+            cmds = get_plugin_commands()
+            assert "cmd-a" in cmds
+            assert "cmd-b" in cmds
+            assert cmds["cmd-a"]["description"] == "A"
+
+    def test_commands_tracked_on_loaded_plugin(self, tmp_path, monkeypatch):
+        """Commands registered during discover_and_load() are tracked on LoadedPlugin."""
+        plugins_dir = tmp_path / "hermes_test" / "plugins"
+        _make_plugin_dir(
+            plugins_dir, "cmd-plugin",
+            register_body=(
+                'ctx.register_command("mycmd", lambda a: "ok", description="Test")'
+            ),
+        )
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path / "hermes_test"))
+
+        mgr = PluginManager()
+        mgr.discover_and_load()
+
+        loaded = mgr._plugins["cmd-plugin"]
+        assert loaded.enabled
+        assert "mycmd" in loaded.commands_registered
+
+    def test_commands_in_list_plugins_output(self, tmp_path, monkeypatch):
+        """list_plugins() includes command count."""
+        plugins_dir = tmp_path / "hermes_test" / "plugins"
+        _make_plugin_dir(
+            plugins_dir, "cmd-plugin",
+            register_body=(
+                'ctx.register_command("mycmd", lambda a: "ok", description="Test")'
+            ),
+        )
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path / "hermes_test"))
+
+        mgr = PluginManager()
+        mgr.discover_and_load()
+
+        info = mgr.list_plugins()
+        assert len(info) == 1
+        assert info[0]["commands"] == 1
+
+    def test_handler_receives_raw_args(self):
+        """The handler is called with the raw argument string."""
+        mgr = PluginManager()
+        manifest = PluginManifest(name="test-plugin", source="user")
+        ctx = PluginContext(manifest, mgr)
+
+        received = []
+        ctx.register_command("echo", lambda args: received.append(args) or "ok")
+
+        handler = mgr._plugin_commands["echo"]["handler"]
+        handler("hello world")
+        assert received == ["hello world"]
+
+    def test_multiple_plugins_register_different_commands(self):
+        """Multiple plugins can each register their own commands."""
+        mgr = PluginManager()
+
+        for plugin_name, cmd_name in [("plugin-a", "cmd-a"), ("plugin-b", "cmd-b")]:
+            manifest = PluginManifest(name=plugin_name, source="user")
+            ctx = PluginContext(manifest, mgr)
+            ctx.register_command(cmd_name, lambda a: a, description=f"From {plugin_name}")
+
+        assert "cmd-a" in mgr._plugin_commands
+        assert "cmd-b" in mgr._plugin_commands
+        assert mgr._plugin_commands["cmd-a"]["plugin"] == "plugin-a"
+        assert mgr._plugin_commands["cmd-b"]["plugin"] == "plugin-b"