chore(ci): replace SourceKnight with setup-sp

[Rushaway]

Summary

• Replace the SourceKnight-based build (maxime1907/action-sourceknight) with a native GitHub Actions workflow.
• Compile with rumblefrog/setup-sp@v1.3.1, pinned to SourceMod 1.12.x.
• Dependencies (multicolors, loghelper, adminhelper, zombiereloaded, EntWatch) are cloned directly from their srcdslab repos and their include files copied into the scripting include path.
• Package and upload the compiled Spectate.smx as a build artifact.
• Auto-tag latest and publish a release on pushes to master/main, and on version tags.
• Remove sourceknight.yaml and the stale .sourceknight gitignore entry, and update .github/copilot-instructions.md to describe the new GitHub Actions build instead of SourceKnight.

Test plan

• Confirm the Build job compiles Spectate.sp successfully in CI
• Confirm the tag/release jobs run on merge to master/main and publish a latest release with the packaged .smx

[Copilot]

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