docs: restructure agent instructions around AGENTS.md

[lukecotter]

📝 PR Overview

Makes AGENTS.md the canonical, single source of agent instructions and reduces CLAUDE.md to a thin import stub, so guidance lives in one place. Adds path-scoped rule files under .claude/rules/ that Claude Code loads on demand per area.

🛠️ Changes made

• Rewrite AGENTS.md as the canonical instructions: monorepo structure, commands, core principles, critical boundary, workflow, and a rules manifest
• Reduce CLAUDE.md to an @AGENTS.md import stub with a pointer to the path-scoped rules
• Add .claude/rules/log-viewer.md (log-viewer/**) covering the vscode-import boundary, performance budgets, key paths, and testing
• Add .claude/rules/lana.md (lana/**) covering extension UX and command paths

🧩 Type of change (check all applicable)

• 🐛 Bug fix - something not working as expected
• ✨ New feature – adds new functionality
• ♻️ Refactor - internal changes with no user impact
• ⚡ Performance Improvement
• 📝 Documentation - README or documentation site changes
• 🔧 Chore - dev tooling, CI, config
• 💥 Breaking change

✅ Tests added?

• 👍 yes
• 🙅 no, not needed
• 🙋 no, I need help

📚 Docs updated?

• 🔖 README.md
• 🔖 CHANGELOG.md
• 📖 help site
• 🙅 not needed