…a-install.sh output to /tmp + copy to /mnt/var/log post-install

Lands B-0834 Approach A (small bounded fix; the operator-side analog
to B-0831 cascade #6 CI workflow-artifact).

Two destinations:
1. /tmp/zeta-install-<UTC-timestamp>.log on live ISO — available for
   `cat | less` after script exits (success OR failure) until reboot;
   also `tail -f` from another tty (Ctrl-Alt-F2) for real-time scrollback
2. /mnt/var/log/zeta-install.log on install target — copied at end of
   script if /mnt is mounted; survives reboot; post-boot inspection via
   `cat /var/log/zeta-install.log | less`

Three banner lines at script-start name the log paths so operator
sees them BEFORE output starts scrolling past.

Enables diagnostic loop for the 5 empirical anchors from the
2026-05-26 physical hardware-support test:
- B-0832 nmtui WiFi rescan failure
- B-0833 interactive-login vs baked-in-keys tension
- B-0835 Bug 1 hostname not unique (control-plane shown)
- B-0835 Bug 2 gh login not respected
- B-0835 Bug 4 self-registration didn't happen (CRITICAL)

Without this fix, the install failures + warnings that scroll past too
fast block diagnosis of Bug 4 (the operator's CORE REQUIREMENT
failure: post-boot fully-operational chain without operator login).

Approach B (script(1) wrapper) deferred — Approach A is sufficient
for the immediate diagnostic need.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>