ci: run Echidna on PRs

[0xCardiE]

Summary

• Add a GitHub Actions Echidna workflow that runs the full fuzz campaign on every pull request (same budget as local yarn echidna: testLimit 60000, seqLen 320).
• One matrix job per harness, 180-minute cap (Echidna --timeout 10200s). Failures upload logs, corpus reproducers, and crytic-export/ as artifacts.
• The Docker runner (yarn echidna) now accepts ECHIDNA_SEED / ECHIDNA_TIMEOUT and tees logs to echidna/logs/.

Echidna image is pinned to ghcr.io/crytic/echidna/echidna:v2.3.1.

Test plan

• Local smoke against the pinned image on EchidnaPriceOracleHarness
• Confirm each harness job on this PR runs the 60k/320 campaign (not the old 4k/80 smoke) and either finishes or hits the 180-minute cap cleanly
• On a property failure, confirm logs + reproducers are uploaded as artifacts

[darkobas2]

Approving. Notes, none blocking:

1. Echidna --timeout 10200s inside a 180-minute job leaves about 10 minutes for checkout, yarn install, hardhat compile --force and the image pull. If that overruns, GitHub kills the job at the cap and the if: failure() upload step is skipped, so you lose exactly the logs you wanted. 9000 would be safer.
2. ECHIDNA_SEED is added for reproducibility but CI never sets one, so "Reproduce a CI counterexample locally" won't actually reproduce. Echidna prints the seed in its output — worth saying in the README to take it from the uploaded log and pass ECHIDNA_SEED=N.
3. No corpus cache between runs, so every PR fuzzes from an empty corpus. actions/cache on echidna/corpus/by-contract/ would let coverage compound. Fine to skip if you want an identical budget per PR.

Checked and correct:

• set +e / PIPESTATUS[0] / set -e captures the fuzzer exit code through the tee.
• scripts/echidna.sh already runs hardhat compile --force on the host, so --hardhat-ignore-compile finds artifacts inside the container.
• echidna/echidna.yaml sets format: text, so no TUI on a non-tty runner.
• pull_request (not pull_request_target) with contents: read — fork PRs get no secrets.

[0xCardiE]

Thnx for notes, adjusted per comments.