CI: Add scheduled nightly Rust toolchain build with auto issue creation

[arvindr19]

Context

PR #3 introduced pr-checks.yml with a pinned nightly toolchain for reproducible PR results.
During review, @lh-sag suggested that tracking latest nightly
Rust changes fits better as a scheduled job rather than a PR check, since unpinned nightly
advances daily and would make PR results non-reproducible.

Goal

Add a scheduled CI workflow that:

• Runs cargo clippy and cargo fmt against the latest nightly toolchain
• Triggers on a cron schedule (e.g. weekly or nightly)
• Automatically opens a GitHub issue when new warnings/errors are found, with details of
  the findings so they can be triaged and fixed before the pinned toolchain is advanced

Considerations

• Will it help developers, or will it create confusion?
• Runner cost: scheduled jobs consume CI minutes independently of PRs, schedule frequency
  should be agreed before implementation
• Auto issue creation: needs a GitHub token with issues: write permission and de-duplication
  logic (avoid opening duplicate issues if a previous one is still open)

Related

• feat: initial CI/CD workflow for uds2sovd-proxy #3 (initial CI/CD workflow)
• check eclipse-opensovd/cicd-workflows for potential shared implementation