vm walkthrough: unlock the tester keychain before the assert phase

[m4ttheweric]

The v2.9.0 walkthrough's assert phase failed on its first run of the new state-backup assertions: assert-installed.sh runs over SSH where the login keychain is locked, so rt state backup init hit security exit 36 and correctly refused to mint an age key blind. The screens phase (walkthrough.sh:222) already unlocks the keychain before its SSH command; the assert invocation (line 243) is the only SSH leg that forgot to. This mirrors the exact idiom from line 222.

The product behaved correctly (refusing to mint when the keychain is unreachable protects an existing key); this is purely a harness-environment fix.

🤖 Generated with Claude Code

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• rt-tray/vm/run/walkthrough.sh

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