chore(ci): replace SourceKnight with setup-sp

[Rushaway]

Summary

Replaces the SourceKnight-based build (maxime1907/action-sourceknight) with a native GitHub Actions workflow that uses rumblefrog/setup-sp to compile the plugin directly with spcomp, following the same job structure already validated in srcdslab/sm-plugin-AdminRoom.

Plugin targets

Target	Source file	Output
FixSvGravity	addons/sourcemod/scripting/FixSvGravity.sp	addons/sourcemod/plugins/FixSvGravity.smx

Dependencies

Name	Repo URL	Unpack path used
sourcemod	https://sm.alliedmods.net/smdrop/1.11/sourcemod-1.11.0-git6917-linux.tar.gz (type: tar)	/addons → /addons (not needed in new CI — replaced by rumblefrog/setup-sp with SourceMod 1.12.x)

This plugin declared no type: git dependencies in sourceknight.yaml, so the new workflow has no dependency-clone step.

Changes

• Added .github/workflows/ci.yml with build / tag / release jobs mirroring the AdminRoom reference (SourceMod pinned to 1.12.x via rumblefrog/setup-sp).
• Removed sourceknight.yaml.
• Updated .github/copilot-instructions.md to describe the native GitHub Actions build instead of SourceKnight.
• Removed the now-unused .sourceknight / .venv entries from .gitignore.
• Packaging step copies the compiled .smx into addons/sourcemod/plugins/ inside the artifact; no configs/, translations/, or other extra directories exist in this repo's tree, so nothing else is packaged.

Not locally validated

This PR was prepared without cloning dependencies or running spcomp locally (lightweight migration pass). The actual compile is not verified locally — this relies on the CI run itself (GitHub Actions build job) to confirm the plugin compiles successfully with SourceMod 1.12.x.

Manual verification needed

• Confirm the CI build succeeds and the produced package layout is correct once Actions run on this PR.
• No configs/, translations/, or other asset directories were found in the repository tree at migration time, so the package step only includes the compiled .smx. Please confirm this is complete once CI runs.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.