feat(audit-guard): setup automated mutation fuzzing (#105)

[codewithzubair07]

• Refactor scanner-engine into lib.rs + main.rs split so fuzz targets can call internal functions without spawning a process
• Expose scan_content() for in-memory fuzzing (no disk I/O)
• Add cargo-fuzz workspace under scanner-engine/fuzz/ with three libFuzzer targets:
  • fuzz_scan_file – drives scan_content() with arbitrary UTF-8
  • fuzz_sha256 – asserts sha256_of() output is always 64-char hex
  • fuzz_rule_regex – runs every compiled regex against arbitrary input
• Add .github/workflows/mutation-fuzzing.yml:
  • Triggers on PR/push to scanner-engine/** and nightly schedule
  • Runs each fuzz target for 30s on PRs, 300s on schedule
  • Uploads crash artifacts (30-day retention) and corpus (7-day)
  • Posts PR comment on crash with local reproduction command
  • Generates job step summary with per-target status

Closes #105