e2e: drive the real agent and file tools through the built webclient ...

New e2e/ suite (outside testpaths, so the normal run never collects it):
a throwaway Postgres database, a real navi-server with an isolated temp
CWD/workspace, the built SPA served by the backend and the production LLM
route from llm.json. Covers filesystem_write write/read, filesystem_write
edit plus filesystem_read diff, and file_ai edit, asserting both the
rendered tool cards and the files on disk.

Each run drops its database; MCP servers stay off because the temp CWD has
no mcp_servers.d, and failures save a screenshot, the DOM and the browser
log. docs/e2e.md documents the harness, requirements and knobs.