fix(notes): keep no file on disk for an empty note

[reybits]

Opening a persistent note was enough to create its file: the save path wrote the buffer unconditionally, so an untouched Local note left a stray .scratch.md in the project (and a global.md plus its directory in stdpath("data")). Clearing a note left the file behind as an empty one.

Write the file only when the note has content, and delete an existing file when the note holds nothing but blank lines.

[Copilot]

🟢 Approval recommended

The change is small, well-scoped, and consistently enforces the intended “no empty note files on disk” behavior without introducing new API surface.

Pull request overview

This PR adjusts scratch.nvim’s persistence behavior so that persistent notes (local/global) don’t leave empty files on disk, aligning file creation/deletion with whether the note actually contains non-blank content.

Changes:

• Update persistence logic to skip writing when the buffer is blank and to delete any existing note file when the buffer becomes blank.
• Add a helper to detect “blank-only” notes (whitespace/empty lines).
• Document the new behavior in the README features list.

File summaries

File	Description
README.md	Documents that empty notes do not create/retain files on disk.
lua/scratch/init.lua	Implements “write only when non-blank; delete file when blank-only” persistence behavior.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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