docs: add agent guidance

[jevin98]

Summary

• Add a root AGENTS.md with project-specific guidance for coding agents.
• Document the WXT + Vue extension structure, pnpm commands, WXT i18n layout, generated-output boundaries, and PR expectations.
• Require pnpm exec eslint . --fix after code generation or code-modifying tasks.
• Keep the small ESLint mechanical formatting fixes produced by the required format run.

Verification

• pnpm install --frozen-lockfile
• pnpm exec eslint . --fix
• pnpm exec wxt prepare
• pnpm exec eslint .
• pnpm compile