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/main.py

@@ -4308,16 +4308,25 @@ For more help on a command:
     # =========================================================================
     mcp_parser = subparsers.add_parser(
         "mcp",
-        help="Manage MCP server connections",
+        help="Manage MCP servers and run Hermes as an MCP server",
         description=(
-            "Add, remove, list, test, and configure MCP server connections.\n\n"
+            "Manage MCP server connections and run Hermes as an MCP server.\n\n"
             "MCP servers provide additional tools via the Model Context Protocol.\n"
-            "Use 'hermes mcp add' to connect to a new server with interactive\n"
-            "tool discovery. Run 'hermes mcp' with no subcommand to list servers."
+            "Use 'hermes mcp add' to connect to a new server, or\n"
+            "'hermes mcp serve' to expose Hermes conversations over MCP."
         ),
     )
     mcp_sub = mcp_parser.add_subparsers(dest="mcp_action")
 
+    mcp_serve_p = mcp_sub.add_parser(
+        "serve",
+        help="Run Hermes as an MCP server (expose conversations to other agents)",
+    )
+    mcp_serve_p.add_argument(
+        "-v", "--verbose", action="store_true",
+        help="Enable verbose logging on stderr",
+    )
+
     mcp_add_p = mcp_sub.add_parser("add", help="Add an MCP server (discovery-first install)")
     mcp_add_p.add_argument("name", help="Server name (used as config key)")
     mcp_add_p.add_argument("--url", help="HTTP/SSE endpoint URL")