feat: add MCP server mode — hermes mcp serve (#3795)

hermes mcp serve starts a stdio MCP server that lets any MCP client
(Claude Code, Cursor, Codex, etc.) interact with Hermes conversations.

Matches OpenClaw's 9-tool channel bridge surface:

Tools exposed:
- conversations_list: list active sessions across all platforms
- conversation_get: details on one conversation
- messages_read: read message history
- attachments_fetch: extract non-text content from messages
- events_poll: poll for new events since a cursor
- events_wait: long-poll / block until next event (near-real-time)
- messages_send: send to any platform via send_message_tool
- channels_list: browse available messaging targets
- permissions_list_open: list pending approval requests
- permissions_respond: allow/deny approvals

Architecture:
- EventBridge: background thread polls SessionDB for new messages,
  maintains in-memory event queue with waiter support
- Reads sessions.json + SessionDB directly (no gateway dep for reads)
- Reuses send_message_tool for sending (same platform adapters)
- FastMCP server with stdio transport
- Zero new dependencies (uses existing mcp>=1.2.0 optional dep)

Files:
- mcp_serve.py: MCP server + EventBridge (~600 lines)
- hermes_cli/main.py: added serve sub-parser to hermes mcp
- hermes_cli/mcp_config.py: route serve action to run_mcp_server
- tests/test_mcp_serve.py: 53 tests
- docs: updated MCP page + CLI commands reference

hermes_cli/mcp_config.py

@@ -608,6 +608,11 @@ def mcp_command(args):
     """Main dispatcher for ``hermes mcp`` subcommands."""
     action = getattr(args, "mcp_action", None)
 
+    if action == "serve":
+        from mcp_serve import run_mcp_server
+        run_mcp_server(verbose=getattr(args, "verbose", False))
+        return
+
     handlers = {
         "add": cmd_mcp_add,
         "remove": cmd_mcp_remove,
@@ -626,6 +631,7 @@ def mcp_command(args):
         # No subcommand — show list
         cmd_mcp_list()
         print(color("  Commands:", Colors.CYAN))
+        _info("hermes mcp serve                              Run as MCP server")
         _info("hermes mcp add <name> --url <endpoint>        Add an MCP server")
         _info("hermes mcp add <name> --command <cmd>         Add a stdio server")
         _info("hermes mcp remove <name>                      Remove a server")