fix(server): t3 service install no longer rebinds another home's service

A CLI run without --base-dir saw a service installed with one as broken,
and `t3 service install` then rewrote the unit for ~/.t3, so the service
came back on an empty data dir. Install now refuses when the unit serves
another T3 home, and `t3 service status` and onboarding name that home
instead of offering a repair.

Closes #23858

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
1baadc63e3PR Batch Tester committed on 10/22/2026, 9:58:14 PMparent2c8bdf6

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