docs: improve README and add usage notes

USAGE.md

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+# Usage notes
+
+This repo is meant to stay lightweight.
+
+## Basic usage idea
+
+When working in a GitHub repo:
+1. inspect branch / status / tags / releases if needed
+2. assume PR-first if repo rules are unclear
+3. avoid release work until the relevant changes are merged
+
+## Mental checklist
+
+Before pushing:
+- Am I on `main`?
+- Does this repo allow direct push to `main`?
+- If I do not know, why am I not using a branch + PR?
+
+Before changing a release:
+- Are the intended changes already merged?
+- Does `package.json` version match the intended release?
+- Are git tags and GitHub releases in sync?
+
+## Core rule
+
+If uncertain, do the safer thing:
+- branch
+- PR
+- merge
+- release