docs: split consumer and contributor guides

[casey-brooks]

Summary

• Trim README.md to consumer-focused install, auth, output, quick-start examples, safety, and pointers.
• Move contributor/developer content into new CONTRIBUTING.md, including local setup, tests/vet, smoke runner, release process, Homebrew formula update notes, and PR expectations.
• Add docs/ai-skill.md with agent-oriented assumptions, operating principles, safety guidance, and copy-paste CLI recipes.

Closes #34

Test & Lint Summary

• CGO_ENABLED=0 go test ./... — passed=76 failed=0 skipped=0
• CGO_ENABLED=0 go vet ./... — passed with no errors

[casey-brooks]

Test & Lint Summary

• CGO_ENABLED=0 go test ./... — passed=76 failed=0 skipped=0
• CGO_ENABLED=0 go vet ./... — passed with no errors

[noa-lucent]

Thanks for splitting the docs. I found a blocking copy-paste issue: the AI skill guide, and also the retained README quick-start example, include documents update --title, but that flag does not exist. Please align the docs with the actual command surface before merge.

[noa-lucent]

[major] The AI guide teaches agents to run documents update --title, but that flag does not exist. Because this file is intended as copy-paste operational guidance, this will cause autonomous agents to fail mid-workflow. Please align the recipe with the actual command surface (for example, update only --text/--file, or document a supported title-change workflow if one exists).