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.

cli.py

@@ -3294,6 +3294,27 @@ class HermesCLI:
         elif cmd_lower == "/reload-mcp":
             with self._busy_command(self._slow_command_status(cmd_original)):
                 self._reload_mcp()
+        elif cmd_lower == "/plugins":
+            try:
+                from hermes_cli.plugins import get_plugin_manager
+                mgr = get_plugin_manager()
+                plugins = mgr.list_plugins()
+                if not plugins:
+                    print("No plugins installed.")
+                    print(f"Drop plugin directories into ~/.hermes/plugins/ to get started.")
+                else:
+                    print(f"Plugins ({len(plugins)}):")
+                    for p in plugins:
+                        status = "✓" if p["enabled"] else "✗"
+                        version = f" v{p['version']}" if p["version"] else ""
+                        tools = f"{p['tools']} tools" if p["tools"] else ""
+                        hooks = f"{p['hooks']} hooks" if p["hooks"] else ""
+                        parts = [x for x in [tools, hooks] if x]
+                        detail = f" ({', '.join(parts)})" if parts else ""
+                        error = f" — {p['error']}" if p["error"] else ""
+                        print(f"  {status} {p['name']}{version}{detail}{error}")
+            except Exception as e:
+                print(f"Plugin system error: {e}")
         elif cmd_lower.startswith("/rollback"):
             self._handle_rollback_command(cmd_original)
         elif cmd_lower == "/stop":