bug: The site search does not stop outside $HOME, and --domain can point outside the home directory #3

Blocker: yes. Fix this before the server-monitoring work starts.

Defect

• FindComposeFile (internal/utils/finder.go:23-35) goes up the directory tree until it gets to $HOME.
• If the current directory is not in $HOME, the search gets to /. filepath.Dir("/") returns /. Thus, the loop does not stop, and the process uses 100% CPU.
• Test: go to a directory outside $HOME that has no docker-compose.yml above it, for example /var/www. Run fly start. Result: the command does not stop.
• The function ignores the errors from os.UserHomeDir() and os.Getwd().
• The function puts the --domain value into a path, but it does not examine the value.
• Test: fly start --domain ../ with HOME=/home/fly. Result: the CLI uses /home/docker-compose.yml.

Cost

• If a user runs a command from the wrong directory, the process does not stop. It uses a full CPU core until the user stops it.
• A systemd service starts in / by default. If agent code calls this function, the agent stops and does not continue.
• The control plane will send domain values. Without a check, a remote value can point to files outside the sites directory. This is a security risk.
• The Cursor rules on main say that domain validation prevents path traversal. This statement is not correct.

Benefit of the fix

• The search always stops. It gives a clear "not found" error.
• The CLI can safely accept a --domain value from the control plane.
• The CLI does not lose errors from the operating system.

Fix

1. Stop the loop when filepath.Dir(dir) == dir.
2. Return the errors from os.UserHomeDir() and os.Getwd().
3. Accept a --domain value only if it is a valid hostname. Reject path separators, .. and empty labels.
4. Make sure that the final path is in the sites root directory.

Completion checks

• A command that starts outside $HOME shows "not found" immediately.
• --domain ../, --domain .. and --domain a/b show a clear error.
• Unit tests for the two items above.