Document the README + HISTORY cspell CI scope in CODESTYLE

CODESTYLE.md

@@ -34,6 +34,7 @@ These apply repo-wide, in every directory:
 
 1. **Markdown**: keep `.md` files clean under a standard markdownlint ruleset, with `MD013` (line-length) treated as disabled - long prose lines are intentional here (one logical paragraph per line). Fix violations at the source rather than disabling rules wholesale.
 2. **Spelling**: US English throughout (the repo-wide convention - see [AGENTS.md](./AGENTS.md)). Project-specific terms (`aiopurpleair`, `PurpleAir`, `aiohttp`, `pydantic`, `syrupy`, `NBGV`) are correct, not misspellings.
+3. **Spelling CI scope**: The enforced CI spell-check gate covers **`README.md` and `HISTORY.md` only** - these are the files every repo visitor sees, so they must be clean. It is deliberately **not** all `**/*.md`: repos carry many markdown files full of technical terms, and gating every one of them would mean endlessly padding `cspell.json` just to keep CI green. Broad, live spell-checking across any file (source, markdown, text) is the **cspell editor extension's** job, so typos still surface to whoever is editing. A repo owner **may** widen their own CI file list, but the template ships README + HISTORY as the default; keep every surface that runs cspell - the CI workflow and any local VS Code task or one-liner the repo has - on the same file list. The list is explicit (not a glob), so a repo that ships no `HISTORY.md` (e.g. one with no changelog) must drop it from all three surfaces and gate on `README.md` alone - cspell errors on a listed file that does not exist. Markdown *linting* (item 1) stays repo-wide `**/*.md` - it does not choke on technical terms.
 
 ## Python