Integrate AGENTS.md safely during init

[itsflaid]

Summary

• create a basic AGENTS.md when missing
• require explicit confirmation before appending DevMap context to an existing file
• preserve existing files, reject symlinks, and keep integration idempotent
• document manual testing and current progress

Verification

• pnpm test:cli (43/43)
• pnpm build:cli
• pnpm build:web
• git diff --check

[itsflaid]

good boy