[codex] clean up root docs and assets

[wesbillman]

Summary

• Move root NIP drafts into docs/nips/ with existing NIP docs.
• Move root image assets into docs/assets/ and update the README logo path.
• Remove the empty root package-lock.json and root placeholder E2E test file.
• Ignore future accidental root npm lockfiles.

Validation

• git diff --cached --check
• pre-commit hook: desktop check, desktop Tauri fmt, mobile format/analyze, Rust fmt
• sq agents review --intent "clean up misplaced root docs/assets and remove accidental root clutter"
• pre-push hook: Rust fmt, desktop Tauri fmt, desktop check, desktop build, mobile test, mobile check, desktop Tauri check, Rust clippy, Rust unit tests