chore(ci): replace SourceKnight with setup-sp

[Rushaway]

Summary

• Replace the SourceKnight-based CI (maxime1907/action-sourceknight) with a native GitHub Actions workflow.
• Compile with rumblefrog/setup-sp@v1.3.1, pinned to SourceMod 1.12.x, and spcomp directly.
• Dependency (sm-plugin-PlayerManager) is now cloned straight from its git repo during the build step instead of resolved via sourceknight.yaml.
• Package step still bundles the compiled plugin .smx alongside the addons/sourcemod/gamedata directory (no configs directory in this repo).
• Auto-release flow preserved: pushes to master/main retag latest, tag pushes and master/main pushes trigger a GitHub release with the packaged archive.
• Removed sourceknight.yaml and its .sourceknight cache entry from .gitignore; updated .github/copilot-instructions.md build/CI references accordingly.

Test plan

• Confirm the Build job compiles Status.sp successfully in GitHub Actions
• Confirm the packaged artifact contains the plugin .smx and gamedata
• Confirm release/tag jobs behave as before on merge to master

[Copilot]

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