Clarify permissions needed for programs.clawdbot.documents

[Clausinho]

Summary

When running nh home switch . with programs.clawdbot.documents pointed at /home/clausi/clawd, the evaluation fails with the following assertions, even though the files exist for the logged-in user:

Failed assertions:
- programs.clawdbot.documents must point to an existing directory.
- Missing AGENTS.md in programs.clawdbot.documents.
- Missing SOUL.md in programs.clawbot.documents.
- Missing TOOLS.md in programs.clawdbot.documents.

Steps to reproduce

1. Add programs.clawdbot.documents = /home/<your-user>/clawd or a similar path that already contains AGENTS.md, SOUL.md, and TOOLS.md.
2. Run nh home switch . from that flake.

Context

The nix daemon runs as one of the nixbld* builders, so it must be able to traverse every parent directory of the workspace path. If /home/<your-user> is mode 0700, builtins.pathExists evaluates to false and the assertions fire before the files can be reached.

Proposal

Document that the daemon needs execute/search permissions on each ancestor directory (or that the workspace must live in a location the daemon already has access to). Linking to an example layout or helper would also be helpful.