Add CONTRIBUTING.md and /renumber-adr skill

[maruiz93]

Summary

• Add CONTRIBUTING.md with social norms for PR reviews: comment resolution belongs to the author, reviewers enforce feedback via "Request
  changes"
• Document that ADRs are immutable point-in-time records and should not be edited after acceptance
• Add /renumber-adr Claude Code skill that detects ADR number collisions against the target branch and renumbers files + all cross-references
  before merge

Motivation

We had no written guidance on PR review etiquette, which can lead to confusion about who resolves comments and how reviewers enforce their
feedback. Similarly, concurrent ADR PRs can collide on numbers — the new skill automates detection and renumbering so contributors don't have to
do it manually.

[ralphbean]

Only question: should we encourage the use of make lint - does pre-commit accomplish the same thing?

[maruiz93]

Only question: should we encourage the use of make lint - does pre-commit accomplish the same thing?

No, they don't do the same thing — but I think they should, so I consolidated them.

Before this change, linting was split across two commands with different scopes:

  Check                    | pre-commit | make lint            
  -------------------------|------------|----------                                                                                               
  check yaml               |     ✓      |    
  fix end of files         |     ✓      |                                                                                                         
  trailing whitespace      |     ✓      |                                                                                                         
  detect private key       |     ✓      |                                                                                                         
  check added large files  |     ✓      |                                                                                                         
  check merge conflicts    |     ✓      |                                                                                                         
  check json               |     ✓      |                                                                                                         
  check toml               |     ✓      |                                                                                                         
  mixed line ending        |     ✓      |                                                                                                         
  ruff (lint)              |     ✓      |     ✓                                                                                                   
  ruff format              |     ✓      |                                                                                                         
  ty check                 |     ✓      |     ✓                    
  bandit                   |     ✓      |                                                                                                         
  gitleaks                 |     ✓      |                      
  actionlint               |     ✓      |                                                                                                         
  go vet                   |            |     ✓
  lint ADR statuses        |            |     ✓                                                                                                   
  lint ADR numbers         |            |     ✓                                                                                                   
  lint ADR frontmatter     |            |     ✓                    
                                                                                                                        

Neither was a superset of the other, so contributors had to know which to run when.

Now all checks are pre-commit hooks, and make lint is just a shortcut for pre-commit run (staged changes). I also added make lint-all for running against every file, though I expect that'll be less commonly used.

[ralphbean]

Awesome! Thank you!