docs: add lab1 submission stub

[shiyanovn]

Goal

Set up SSH commit signing and add a PR template for Lab 1.

Changes

• Configured SSH commit signing in git.
• Added minimal pull request template at .github/pull_request_template.md.
• Created labs/submission1.md with answers and screenshots.

Testing

• Created a signed commit, checked that it is marked "Verified" on GitHub.
• Opened a new pull request and confirmed that the template appears automatically.

Checklist

• Clear, descriptive PR title
• Documentation/README updated if needed
• No secrets or large temporary files committed