🐛 Restore Deno 2 compatibility

[AnandChowdhary]

Summary

• Replace removed Deno.run() with Deno.Command so the CLI type-checks and runs under Deno 2.
• Add a local custom-registry regression test that executes a fetched CLI.
• Consolidate and modernize the Deno CI workflow, including pull-request coverage.

Validation

• deno test --allow-run --allow-net (8 passed)
• deno fmt --check mod.ts mod_test.ts
• deno lint mod_test.ts
• Workflow YAML parsing and actionlint
• Independent staged-diff review

Release note

The repository has no configured Actions secrets, including NPM_TOKEN; this PR is intentionally left unmerged until publishing credentials are restored, because this :bug: release-relevant change must ship rather than leave default-branch release CI red.