ci: add shell script linting workflow

[aniketpati1121]

fixes #3703

[google-oss-prow]

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[Copilot]

Pull request overview

This PR adds repo-wide shell script formatting (shfmt) and linting (ShellCheck) enforcement via Make targets, pre-commit hooks, and a dedicated GitHub Actions workflow, aligning this repository with Kubeflow’s CI standards (per #3703).

Changes:

• Add make shell-fmt (shfmt) and make shell-lint (shellcheck) targets plus tool version pinning in the Makefile.
• Add local pre-commit hooks to run the new shell formatting and lint targets.
• Add a new Shell Lint GitHub Actions workflow to enforce shell formatting and linting on PRs and pushes.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
Makefile	Adds shfmt/shellcheck version pins, Make targets, and a ShellCheck download helper.
.pre-commit-config.yaml	Adds local pre-commit hooks to run make shell-fmt and make shell-lint.
.github/workflows/shell-lint.yaml	Adds CI workflow to enforce shell formatting and run ShellCheck.