feat(lab3): SSH signing + gitleaks pre-commit + history rewrite practice

[lashmanovSergey]

Goal

Set up SSH commit signing, integrate gitleaks as a pre-commit hook, and prevent accidental secret leaks.

Changes

• Created .pre-commit-config.yaml with gitleaks (v8.30.1), detect-private-key, and check-added-large-files hooks
• Updated submissions/lab3.md with documentation and evidence

Testing

• git log --show-signature -1 confirms a valid "git" signature from csgosteam888@mail.ru
• Commit 11f4715 shows green Verified badge on GitHub
• Gitleaks successfully blocked a test commit containing a GitHub PAT (RuleID: github-pat)

Artifacts & Screenshots

• submissions/lab3.md contains all required documentation and screenshots

PR checklist body:

• Task 1 — SSH signing configured + Verified badge on commit
• Task 2 — .pre-commit-config.yaml + gitleaks demonstrably blocking
• Bonus — filter-repo rewrite practice documented