Flexible robot_name resolution: literal, hostname, or file

[Minipada]

Parent

#440

What to build

Records already carry robot_name through custom keys, but the value has to be written
literally into each robot's params file. In a fleet that is one hand-edited file per robot.

Make the resolution flexible: a literal name, the machine's hostname, or the contents of a
file, so identity can come from whatever the provisioning process already produces.

Acceptance criteria

• robot_name accepts a literal value (today's behaviour, unchanged)
• robot_name can resolve from the hostname
• robot_name can resolve from the contents of a file
• Hostname is the default when nothing is configured
• The resolved value reaches Records exactly as a literal custom key does today
• Resolution failure (missing file, empty value) is reported clearly at startup, not silently

Blocked by

• None — can start immediately

[Minipada]

Merged into jazzy via #456. Not auto-closed because #456 merged into jazzy, not this repo's default branch (humble).