feat(mcp): .mcp.json config support and disconnect action in /mcp page

[yishuiliunian]

Summary

• Support industry-standard .mcp.json config files in .loopal/ and plugin directories (camelCase mcpServers format, stdio implicit)
• Add disconnect/reconnect action menu in /mcp sub-page (Enter opens menu instead of direct reconnect)
• Full disconnect pipeline: protocol → runtime → MCP manager → tool registry cleanup

Changes

Config layer (loopal-config):

• New mcp_json.rs: parser for standard .mcp.json format
• loader.rs: load_layer_from_dir loads .mcp.json after settings.json, overrides by name
• 12 unit + integration tests

Protocol layer (loopal-protocol):

• ControlCommand::McpDisconnect variant + serde roundtrip test

MCP layer (loopal-mcp):

• disconnect_connection() on McpManager — returns removed tool names for registry cleanup

Kernel layer (loopal-kernel):

• unregister_tools() on Kernel
• ToolRegistry::unregister() method

Runtime layer (loopal-runtime):

• handle_mcp_disconnect() handler: disconnect → unregister tools → emit status

TUI layer (loopal-tui):

• ActionMenu state model with dynamic options (Disconnect shown only when connected)
• Two-mode key handler (list navigation vs menu selection)
• mcp_action_menu.rs: centered overlay rendering
• Status update clears stale action menu
• tool_approve/tool_deny extracted to key_dispatch_ops.rs (line limit fix)
• Updated existing tests + 6 new tests

Test plan

• CI passes
• bazel build //... --config=clippy zero warnings
• bazel build //... --config=rustfmt zero diffs
• 5 affected crate test suites pass (config, protocol, mcp, registry, tui)