feat(mcp): simplify external MCP server config to local/remote modes

Replace the three-way transport choice (stdio / sse / httpstream) for
connecting LangBot to external MCP servers with two modes: local (stdio)
and remote. Remote servers only require a URL; the runtime auto-detects
the transport (tries Streamable HTTP, falls back to SSE).

- provider/tools/loaders/mcp.py: add _init_remote_server() with
  Streamable-HTTP-then-SSE probing; dispatch 'remote' lifecycle, keep
  legacy sse/http branches for back-compat
- plugin/connector.py: normalize legacy http/sse marketplace modes to
  'remote' on Space install, preserving connection params
- entity/persistence/mcp.py: document mode as stdio, remote (legacy: sse, http)
- alembic 0006: idempotent data migration mapping existing sse/http rows
  to remote (downgrade maps back to http)
- api/http/service/mcp.py: stash runtime_info (status + tool list) into
  test task metadata before tearing down the temp session
- web: collapse mode dropdown to local/remote, remote renders URL+timeout
  only, edit auto-maps legacy sse/http to remote; show tools after test in
  create mode from task metadata; remove dead plugins/mcp-server/ tree
- i18n: local/remote labels + mode/url hints across 8 locales

web/src/i18n/locales/en-US.ts

@@ -754,6 +754,15 @@ const enUS = {
     stdio: 'Stdio Mode',
     sse: 'SSE Mode',
     http: 'HTTP Mode',
+    local: 'Local (Stdio)',
+    remote: 'Remote',
+    localModeDescription:
+      'Run an MCP server locally as a subprocess inside the Box sandbox.',
+    remoteModeDescription:
+      'Connect to a remote MCP server by URL. The transport (Streamable HTTP or SSE) is detected automatically.',
+    remoteUrlPlaceholder: 'https://example.com/mcp',
+    remoteUrlDescription:
+      'Paste the MCP server URL. Both Streamable HTTP and legacy SSE endpoints are supported.',
     noServerInstalled: 'No MCP servers configured',
     serverNameRequired: 'Server name cannot be empty',
     commandRequired: 'Command cannot be empty',