docs: Clarify first boot registration service

[m-horky]

The original text roughly contained the steps to set up a service to register with on the next (first) boot, but contained some minor issues. Aside from typos/phrasing:

• It talked of 'startup' and 'next boot', but effectively this would only get executed on first boot.
• Command touch .run_next_boot was contained twice.

[gemini-code-assist]

Code Review

This pull request clarifies the documentation for a first-boot registration service. It improves the wording, fixes typos, removes a duplicated command, and corrects a significant bug in the example systemd service where a wrong path was used in ConditionPathExists. These changes greatly improve the clarity and correctness of the documentation. I have added one comment with a suggestion to further improve the robustness of the systemd service example, ensuring it can retry on failure and that credentials are securely cleaned up.

[gemini-code-assist]

There are a couple of issues with the current service definition that could lead to unexpected behavior:

1. Registration retry: Using ExecStartPre to remove the condition file (.register-on-first-boot) means that if the registration in ExecStart fails (e.g., due to a transient network error), the service won't attempt to register again on the next boot. The condition file should only be removed upon successful registration.

2. Credential cleanup: For a Type=oneshot service without RemainAfterExit=yes, ExecStop is not executed when the ExecStart command completes. This means the credentials file (/etc/management-client/.credentials) will not be removed as intended, leaving secrets on the disk.

A more robust approach is to handle both the conditional file removal and credential cleanup within a single ExecStart command. The suggested change below implements this. The trap command ensures /etc/management-client/.credentials is removed when the shell exits, regardless of success or failure. The && ensures /etc/management-client/.register-on-first-boot is only removed if the registration command is successful.

Suggested change

	ExecStartPre=/bin/rm -f /etc/management-client/.register-on-first-boot
	ExecStart=/usr/bin/management-client register --activation-key ${CLIENT_ACTIVATION_KEY}
	ExecStartPre=/bin/rm -f /etc/management-client/.run_next_boot
	ExecStop=/bin/rm -f /etc/management-client/.credentials
	ExecStart=/bin/sh -c "trap '/bin/rm -f /etc/management-client/.credentials' EXIT; /usr/bin/management-client register --activation-key ${CLIENT_ACTIVATION_KEY} && /bin/rm -f /etc/management-client/.register-on-first-boot"

[m-horky]

This feedback is valid, but I didn't want to alter the existing behavior. For an example, I believe the current version is sufficient.

[henrywang]

Just make a rebase for test install failure.