feat(examples): postgres-backed MCP server with filesystem authorization

[rudi193-cmd]

Summary

Adds examples/servers/postgres-knowledge-server — a production-grade MCP server that demonstrates patterns not covered by existing examples:

Filesystem-based authorization middleware

def authorized(app_id: str) -> bool:
    folder = SAFE_ROOT / app_id
    return folder.is_dir() and (folder / "manifest").exists()

No ACL database. The filesystem shape IS the permission. Grant by creating a folder, revoke by deleting it.

Postgres backend via Unix socket
No host/port exposure. Connects via psycopg2.connect(dbname=..., user=...) on the local Unix socket — zero network surface.

Three tools: knowledge_put, knowledge_get, knowledge_search (Postgres FTS with plainto_tsquery).

stdio-only transport — no HTTP server, no open ports.

This pattern comes from production use in willow-mcp, an agent memory system running this architecture in daily use.

Files

• mcp_postgres_server/server.py — server implementation
• mcp_postgres_server/__main__.py — entry point
• pyproject.toml — package config matching existing examples
• README.md — usage and architecture docs