feat: first-class plugin architecture (#1555)

Plugin system for extending Hermes with custom tools, hooks, and
integrations — no source code changes required.

Core system (hermes_cli/plugins.py):
  - Plugin discovery from ~/.hermes/plugins/, .hermes/plugins/, and
    pip entry_points (hermes_agent.plugins group)
  - PluginContext with register_tool() and register_hook()
  - 6 lifecycle hooks: pre/post tool_call, pre/post llm_call,
    on_session_start/end
  - Namespace package handling for relative imports in plugins
  - Graceful error isolation — broken plugins never crash the agent

Integration (model_tools.py):
  - Plugin discovery runs after built-in + MCP tools
  - Plugin tools bypass toolset filter via get_plugin_tool_names()
  - Pre/post tool call hooks fire in handle_function_call()

CLI:
  - /plugins command shows loaded plugins, tool counts, status
  - Added to COMMANDS dict for autocomplete

Docs:
  - Getting started guide (build-a-hermes-plugin.md) — full tutorial
    building a calculator plugin step by step
  - Reference page (features/plugins.md) — quick overview + tables
  - Covers: file structure, schemas, handlers, hooks, data files,
    bundled skills, env var gating, pip distribution, common mistakes

Tests: 16 tests covering discovery, loading, hooks, tool visibility.

tests/hermes_cli/test_commands.py

@@ -13,6 +13,7 @@ EXPECTED_COMMANDS = {
     "/undo", "/save", "/config", "/cron", "/skills", "/platforms",
     "/verbose", "/reasoning", "/compress", "/title", "/usage", "/insights", "/paste",
     "/reload-mcp", "/rollback", "/stop", "/background", "/skin", "/voice", "/quit",
+    "/plugins",
 }