agentbox apply --dry-run tracebacks for a non-root user, and the shipped guide tells the agent to run it

[defangdevs]

Problem

/etc/agent-box-guides/AGENTS.<user>.md tells the agent:

agentbox apply --dry-run prints what the box's declared configuration would change without changing it - the fastest way to see how this host is actually put together.

An agent is not root, and --dry-run dies on the first file it cannot read:

$ python3 bin/agentbox apply --dry-run --config tests/native/config.json \
    --profile /nix/var/nix/profiles/agent-box
would write /etc/agent-box/units/robot.env
Traceback (most recent call last):
  ...
  File "bin/agentbox", line 3157, in commit
    if write_if_changed(path, text, mode, args.dry_run):
  File "bin/agentbox", line 2748, in write_if_changed
    current = path.read_text()
PermissionError: [Errno 13] Permission denied: '/etc/agent-box/web-users'

write_if_changed reads the current file to decide whether anything differs, and only FileNotFoundError is caught:

    try:
        current = path.read_text()
        same = current == text and (path.stat().st_mode & 0o7777) == mode
    except FileNotFoundError:
        same = False

/etc/agent-box/web-users is 0600 root, so the read raises PermissionError and the run ends mid-way through the file list. Nothing was going to be written -- the crash is entirely in the comparison a dry run does not need to be able to make.

Impact

Read-only inspection of a native box is unavailable to the agent living on it, and the failure looks like a broken renderer rather than a permission the caller does not have. Also true of a partial --dry-run as root on a box where an administrator made one of our files unreadable.

Not a live-apply bug: an apply that runs as root reads these files fine.

Suggested fix

Treat an unreadable file as "differs" rather than as fatal -- catch OSError (and UnicodeDecodeError, which generated_by_us already handles for the same reason) around the comparison:

    except (OSError, UnicodeDecodeError):
        same = False

A dry run then reports would write /etc/agent-box/web-users, which is the honest answer: it cannot prove the file already matches. A real apply as root is unaffected, since it can read the file.

The remaining question is whether a non-root --dry-run should say so up front -- the file list it prints is a superset of what a root run would change, because everything it could not read reads as changed. A one-line note when os.geteuid() != 0 would keep that from being mistaken for drift.

Notes

Spotted while working #526 (PR #539), which is where the run above came from; unrelated to that change and present before it.