feat(host-mcp): serve Executor configuration tools on the passthrough surface

[fforres]

What changes for users

An agent connected with ?mode=passthrough can now add an integration when asked. It can search for and invoke Executor's own configuration tools, such as executor.mcp.addServer, executor.connections.createHandoff and executor.oauth.probe / executor.oauth.start. Before this change, passthrough hid these tools, so "add this MCP server" was only possible from the dashboard, the CLI or codemode.

How

• search no longer drops static tools. It resolves each match to the listed Tool instead of re-parsing a five-segment address, because static tools are addressed by their fqid (executor.mcp.addServer).
• invoke accepts an fqid: when the id is not a five-segment address, it looks the tool up under its integration.
• The search-invoke guide now explains the add-an-integration flow and says to send the user the handoff or OAuth URL, never to collect credentials in chat.

Safety

• Block policies still hide and refuse these tools (tools.list and tools.schema already apply them).
• Org-owned writes still require org write access.
• As with every passthrough invoke, approval belongs to the client (destructiveHint: true).

Tests

passthrough-tools.test.ts: a static executor.mcp.addServer fixture is returned by search with its account details and schema, invoke runs it by fqid, and an unknown fqid is refused. All packages/hosts/mcp tests pass (257).