docs: add agent coding standards

[Tootam]

ELI5

This PR adds small "read this first" notes for future coding agents working in this repo.

The root AGENTS.md explains the shared project rules. Each important folder also gets its own AGENTS.md with local guidance, for example backend safety rules, frontend Vue conventions, static asset handling, and style rules.

Think of these files like labels on workshop drawers: before changing something, an agent can quickly see what belongs there, what patterns to follow, and what not to touch accidentally.

Motivation

Agent-assisted changes are more reliable when the repository explains its expectations directly in the source tree. This project has several important local conventions:

• Deno formatting uses 4-space indentation, semicolons, double quotes, and a wide line length.
• Backend code relies on Hono routes, Zod validation, filesystem-backed tab metadata, and careful filename/auth checks.
• Frontend code uses Vue 3, Vite, Bootstrap/Bootstrap Vue Next, FontAwesome, and alphaTab integration.
• Public font and soundfont folders contain vendored/binary assets that should not be casually changed.

Without scoped guidance, a future automated change can easily drift from these conventions or touch generated/runtime assets by mistake. These docs make the expected practices explicit and local to the directories where they matter.

What Changed

• Added a root AGENTS.md with repository-wide coding standards and verification guidance.
• Added backend-specific guidance for routes, validation, auth/privacy, persistence, and tests.
• Added frontend-specific guidance for Vue/Vite conventions, API helpers, UX expectations, styles, pages, layouts, components, and translations.
• Added asset-specific guidance for public assets, Bravura fonts, and Sonivox soundfonts.
• Added extra/AGENTS.md for build scripts, Docker helpers, demo/template files, and packaging utilities.

Validation

• Confirmed each maintained directory has a scoped AGENTS.md.
• Checked the new Markdown files for non-ASCII characters.
• deno fmt --check was not run because deno is not available in this environment.