feat(mcp): persist and display marketplace README

Capture the README markdown from LangBot Space when installing an MCP
server and store it on the mcp_servers record (new readme column +
alembic migration 0004). The detail page can then render docs offline,
independent of the server's runtime/connection state.

src/langbot/pkg/entity/persistence/mcp.py

@@ -11,6 +11,10 @@ class MCPServer(Base):
     enable = sqlalchemy.Column(sqlalchemy.Boolean, nullable=False, default=False)
     mode = sqlalchemy.Column(sqlalchemy.String(255), nullable=False)  # stdio, sse, http
     extra_args = sqlalchemy.Column(sqlalchemy.JSON, nullable=False, default={})
+    # Markdown documentation captured from LangBot Space at install time so the
+    # detail page can show docs even when the server is offline / has no tools.
+    # Empty string for manually-created servers that have no marketplace README.
+    readme = sqlalchemy.Column(sqlalchemy.Text, nullable=False, server_default='', default='')
     created_at = sqlalchemy.Column(sqlalchemy.DateTime, nullable=False, server_default=sqlalchemy.func.now())
     updated_at = sqlalchemy.Column(
         sqlalchemy.DateTime,

src/langbot/pkg/persistence/alembic/versions/0004_add_mcp_readme.py

@@ -0,0 +1,34 @@
+"""add readme column to mcp_servers
+
+Revision ID: 0004_add_mcp_readme
+Revises: 0003_add_rerank_models
+Create Date: 2026-06-06
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = '0004_add_mcp_readme'
+down_revision = '0003_add_rerank_models'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Add ``readme`` to mcp_servers if the table exists and the column is missing
+    # (the table may have been created by create_all() with the column already
+    # present on fresh installs, so guard against duplicate-add).
+    conn = op.get_bind()
+    inspector = sa.inspect(conn)
+    if 'mcp_servers' not in inspector.get_table_names():
+        return
+    columns = {col['name'] for col in inspector.get_columns('mcp_servers')}
+    if 'readme' not in columns:
+        op.add_column(
+            'mcp_servers',
+            sa.Column('readme', sa.Text(), nullable=False, server_default=''),
+        )
+
+
+def downgrade() -> None:
+    op.drop_column('mcp_servers', 'readme')

src/langbot/pkg/plugin/connector.py

@@ -248,6 +248,9 @@ class PluginRuntimeConnector(ManagedRuntimeConnector):
 
         mode = mcp_data.get('mode') or 'stdio'
         extra_args = mcp_data.get('extra_args') or {}
+        # 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 ''
         # Use __ instead of / to avoid URL routing issues with slashes
         name = f'{mcp_data.get("author", "")}__{mcp_data.get("name", "")}'
 
@@ -267,6 +270,7 @@ class PluginRuntimeConnector(ManagedRuntimeConnector):
             'enable': True,
             'mode': mode,
             'extra_args': extra_args,
+            'readme': readme,
         }
 
         await self.ap.persistence_mgr.execute_async(sqlalchemy.insert(persistence_mcp.MCPServer).values(server_data))