fix(init): remove stale head sha from CI snippets

[zeitlinger]

Summary

Stops generated CI snippets and docs from exporting GITHUB_HEAD_SHA, which Rust Flint no longer reads.

The generated workflow now passes the GitHub link-remap environment Flint actually uses: GITHUB_REPOSITORY, GITHUB_BASE_REF, GITHUB_HEAD_REF, PR_HEAD_REPO, and GITHUB_TOKEN, while keeping fetch-depth: 0.

The missing-env failure/warning is intentionally deferred.

Validation

• cargo fmt
• cargo test generate_lint_workflow
• mise run lint:fix
• git diff --check

[Copilot]

Pull request overview

Removes the now-unused GITHUB_HEAD_SHA from generated CI snippets and updates the scaffolded/generated lint workflow to pass the GitHub env vars Flint’s link-remap logic actually consumes.

Changes:

• Update the init scaffold template for .github/workflows/lint.yml to stop exporting GITHUB_HEAD_SHA and export GITHUB_REPOSITORY, GITHUB_BASE_REF, GITHUB_HEAD_REF, and PR_HEAD_REPO instead (plus GITHUB_TOKEN).
• Extend the init workflow-generation test to assert the new env vars are present and GITHUB_HEAD_SHA is absent.
• Update README and this repo’s lint workflow to match the new recommended env set.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/init/tests.rs	Adds assertions for the new workflow env vars and verifies GITHUB_HEAD_SHA is no longer emitted.
src/init/scaffold.rs	Updates the generated lint.yml template env block to match Flint’s current GitHub link-remap inputs.
README.md	Updates the CI snippet and explanation to remove GITHUB_HEAD_SHA and document the new env vars.
.github/workflows/lint.yml	Aligns the repo’s own lint workflow with the new env vars used by Flint.

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