Open source release

[JesseHerrick]

Prepares the repo for our open source release:

• Replace all repo links with our GitHub link
• Add a GitHub Actions CI flow
• Update README

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Note

Medium Risk
Mostly repo/CI/documentation changes, but it also adjusts LSP background reindex goroutine lifecycle tracking, which could affect shutdown/cleanup behavior under concurrency.

Overview
Prepares the project for an open-source GitHub release: switches the Go module/import paths from GitLab to github.com/remoteoss/dexter, adds an MIT LICENSE, and refreshes README.md with GitHub-based install/editor/docs guidance plus new GitHub issue templates.

Replaces GitLab CI with GitHub Actions, adding a CI workflow (unit, integration, lint) and a tag-based Release workflow that builds multi-platform binaries, uploads artifacts, and publishes a GitHub release with checksums.

Updates distribution tooling: the asdf plugin lib/utils.bash now prefers downloading prebuilt GitHub release tarballs (with a source-build fallback), removes bin/check-deps, and updates tests/builds to use -buildvcs=false. Also adds backgroundWork tracking to the LSP server to ensure background reindex goroutines finish before the store is closed (tests now Wait() on cleanup).

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