docs: align contributor guidelines with divine-mobile

[NotThatKindOfDrLiz]

Summary

• Rewrite AGENTS.md in the section structure used in the divine-mobile repo (Repo Shape, Worktree-First Workflow, PR Guardrails, No Tech Debt, Architecture, OAuth/Signing/Identity Rules, Verification, Database, Secrets/Deployment, Clean Workspace), adapted to keycast's Rust + SvelteKit stack and actual commands.
• Add a new CONTRIBUTING.md modeled on divine-mobile's outside-contributor guide: current contribution status, good first contributions, before-you-start, templates/agent compliance, technical-debt standard, why PRs get closed, repo setup, worktree-first workflow, where-to-work crate map, architecture and security boundaries, day-to-day commands, testing, scope discipline, PR checklist, secrets, docs rules.

Avoid naming corporate partners, customers, or other sensitive external brands in this PR title or body. Use generic descriptors unless a maintainer explicitly approves the public reference.

Motivation

• The previous AGENTS.md was a short summary that did not match the depth or structure of the divine-mobile repo's contributor guidance, and there was no CONTRIBUTING.md for outside contributors at all.
• Aligning the structure across repos makes it easier for both humans and coding agents to work across keycast and divine-mobile without re-learning the workflow each time.
• Codifies the worktree-first workflow, no-tech-debt standard, and OAuth/signing/identity sensitivity rules that were already expected but not written down here.

Related Issue

• Closes #

Testing

• Docs-only change; no code paths exercised.
• cargo test --workspace --verbose
• cargo clippy --workspace --all-targets --all-features -- -D warnings -A deprecated
• cargo fmt --all -- --check
• Manual verification: re-read both files end-to-end; confirmed referenced commands match package.json scripts and referenced paths exist (.github/pull_request_template.md, .github/workflows/semantic_pr.yml, .github/ISSUE_TEMPLATE/, docs/, CLAUDE.md).

Visuals

• UI/web change with screenshots/video attached
• No visual change
• Visuals and text avoid sensitive external brand or partner names unless explicitly approved

🤖 Generated with Claude Code

[dcadenas]

✅ contributor docs match mobile flow