docs(copilot-instructions): clean up merge markers and stale facts

[CodeLieutenant]

Summary

.github/copilot-instructions.md had accumulated three sets of unbookended
git merge markers (||||||| <ref> and ======= without their <<<<<<< /
>>>>>>> counterparts) plus the same sections duplicated 3-4× from botched
merges or cherry-picks. Codex/Copilot couldn't usefully consume it.

This PR rewrites the file to a single consolidated version and corrects a
few facts that had drifted from current state:

• Resolved all conflict markers and deduplicated the Testing /
  Project Structure / Benchmarking Modes / Building and Running /
  Development Workflow / Troubleshooting sections.
• Go 1.24 → 1.25 to match go.mod.
• gocql replace target v1.15.0 → v1.18.0 to match go.mod (PR chore(deps): update module github.com/scylladb/gocql to v1.18.0 #287
  bumped this and the doc was never updated).
• Refreshed the pkg/ tree to list the packages that actually exist
  today (config, ratelimiter, results, stack, testrun,
  testutil, tools, worker, workloads) plus internal/clock with
  a note that time.Now/Sleep should not be called directly.
• Dropped the "make check is broken — don't run it" warning.
  .golangci.yml and the Makefile target both look healthy now; the
  guidance was steering contributors away from a working local lint
  check. Reinstated make check in the pre-commit checklist.
• Added the missing make build-sct-docker-image, make fieldalign,
  and the integration test entrypoints (TestIntegration*,
  TestMemoryLeak*) with their env-var gates.

[fruch]

I would recommend splinting this to two PRs

[CodeLieutenant]

I would recommend splinting this to two PRs

Splitting it into two PRs does not much sense, since these skills are from skills.sh and they are installed and managed with a package manager, It would be even safe to gitignore them and then everybody who wants to work on s-b should install it as a part of some kind of setup phase, but everybody will forget that, so it's much easier just to vendor it and leave it in the source, since it will be updated with skills.sh cli, and everybody will benefit

[soyacz]

this has nothing to do with scyla-bench
We could think of separate scylla repo for these or posting it to proper slack channel for others to use

[soyacz]

I see many skills installed, did you have a chance to implement something with their use? won't they clash or make agent to load too many of them?

[CodeLieutenant]

I see many skills installed, did you have a chance to implement something with their use? won't they clash or make agent to load too many of them?

I did not test it with scylla bench, but I did in gemini. For quite some time gemini has exact skills, and they work great. LLMs are smart enough to load only whats needed (at least with opencode and Claude, haven't tested others).

They do not interfere with each other and no overlap from what I've seen and based on whats in there.