ci: run the performance gate nightly with retries

[tinder-maxwellelliott]

Summary

• Move the Kotlin vs Rust performance gate off pull requests and master pushes. It now runs on a nightly cron (0 7 * * *, 07:00 UTC) and on workflow_dispatch.
• Cap each measurement attempt at one hour, retry up to three times, and keep the job budget large enough for setup/build plus those attempts (timeout-minutes: 210).
• Update the README and gate docs so they no longer claim the check blocks every PR.

Test plan

• Confirm the workflow YAML still parses and no longer lists push / pull_request triggers.
• After merge, trigger Performance Gate from the Actions tab (workflow_dispatch) and confirm a hung/timed-out attempt retries, a passing attempt stops the loop, and a persistent failure still fails the job.
• Confirm a subsequent PR does not start the Performance Gate workflow.