docs: add agentic workflow project documentation

[britle20]

Summary

Adds repository documentation and workflow files for an agentic Vite + React + TypeScript mini DAW workflow.

Changes

• Added AGENTS.md, README updates, PLANS.md, and docs for product, architecture, data model, audio engine, UI, and testing.
• Added feature specs for project scaffold, sample playback, lookahead scheduler, drum sequencer, and piano roll work.
• Added GitHub CI, PR template, issue templates, and Vite-oriented .gitignore.

Testing

• Required file tree check passed.
• Markdown internal reference check passed.
• git diff --cached --check passed before commit.
• npm script checks were not applicable because package.json does not exist yet.

Notes / Limitations

• The Vite scaffold is intentionally not implemented in this PR.
• CI skips npm install and script checks until package.json exists, then uses --if-present for documented scripts.
• Initial feature issues will track the first implementation tasks separately.