docs: adopt Linux-style contribution workflow with email patches

[LunaStev]

Restructure contribution guidelines to support both GitHub PRs and email-based patch submissions with automated verification.

Changes:

• Update .mailmap to use unified email address (luna@lunastev.org):
  • Consolidate all contributor identities under single canonical address
  • Maintain historical attribution for previous email addresses
• Rewrite CONTRIBUTING.md with kernel-style workflow:
  • Add dedicated development setup section (references wavfnd/setup)
  • Document dual contribution methods (GitHub PR + email patches)
  • Require DCO (Developer Certificate of Origin) for all commits
  • Add patch submission instructions with git format-patch/send-email
  • Simplify project scope section (no builtin functions philosophy)
  • Remove verbose "beginner-friendly" content and community fluff
  • Focus on technical requirements and maintainer workflows
• Add MAINTAINERS file with subsystem ownership:
  • Define maintainers for frontend (lexer, parser)
  • Assign error system, LLVM backend, and CLI ownership
  • Use email addresses for patch routing
  • Follow F: (file pattern) and M: (maintainer) format
• Implement get_maintainer.py script:
  • Parse MAINTAINERS file to extract subsystem information
  • Match file paths to appropriate maintainer email addresses
  • Support batch file input for patch series
  • Print CC list for manual patch submission
• Add verify_patch.sh automated verification script:
  • Apply patch with git am in temporary branch
  • Verify DCO signature presence
  • Run cargo fmt --check for style compliance
  • Execute cargo build, cargo test, and cargo clippy
  • Clean up temporary branch on success or failure
  • Exit with error code if any check fails

This establishes a professional contribution workflow similar to Linux kernel and LLVM projects, enabling both GitHub-based and email-based patch submission with maintainer-driven review process.