refactor(server): trim the base-dir fix to the install guard

Drop the symlink handling, the fail-closed unit read, the onboarding
message, the docs paragraph and the extra tests. What stays is the fix
for #23858: install refuses a unit that serves another T3 home, and
status names that home.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
a24df3e992PR Batch Tester committed on 10/23/2026, 8:31:59 AMparentdefd523

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