Instrument guest exec steps

[sjmiller609]

Summary

• Adds detailed guest.exec child spans when command execution is waiting for the guest agent to become reachable.
• Breaks the wait path into connection lookup, exec stream open, start request send, and receive-until-exit timing.
• Records receive-path byte counts, bytes sent, and exit code so traces can distinguish first-contact/setup cost from in-guest command runtime.
• Keeps the no-wait exec path free of extra guest.exec.* spans.
• Makes the Linux test subnet lock reopen an existing shared lock file safely so parallel integration tests can coordinate reliably.

Tests

• go test ./lib/guest -count=1
• CI: test, test-darwin, e2e-install, preview, semgrep, socket, and bugbot are passing.

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