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

src/langbot/pkg/plugin/connector.py

@@ -248,6 +248,15 @@ class PluginRuntimeConnector(ManagedRuntimeConnector):
 
         mode = mcp_data.get('mode') or 'stdio'
         extra_args = mcp_data.get('extra_args') or {}
+        # The MCP transport selection was simplified to two modes: 'stdio'
+        # (local, Box-sandboxed) and 'remote' (the runtime auto-detects
+        # Streamable HTTP vs. legacy SSE from the URL). Marketplace records may
+        # still carry the older 'http'/'sse' modes — normalize them to 'remote'
+        # so the installed server shows up correctly in the two-option UI. The
+        # connection args (url/headers/timeout/ssereadtimeout) are preserved and
+        # consumed by the auto-detecting remote transport regardless.
+        if mode in ('http', 'sse'):
+            mode = 'remote'
         # Marketplace records carry the rendered README markdown; persist it so
         # the detail page Docs tab works offline and without a marketplace round-trip.
         readme = mcp_data.get('readme') or ''