Document the README + HISTORY cspell CI scope (main-only) (#517)

Cherry-picks the CODESTYLE "Spelling CI scope" documentation onto main,
without dragging the unrelated in-flight work currently on develop.
Docs-only; the same change is already on develop.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: ptr727

CODESTYLE.md

@@ -35,6 +35,7 @@ These apply repo-wide, in every directory:
 
 1. **Markdown linting**: All `.md` files must be lint-clean (error and warning free) via the VS Code `markdownlint` extension. [`.markdownlint-cli2.jsonc`](./.markdownlint-cli2.jsonc) at the repo root is the single source of truth - the davidanson `markdownlint` extension and a command-line `markdownlint-cli2` run both read it, so the IDE and CLI stay in lock-step. Rules it deliberately disables (e.g. `MD013` line-length, `MD033` inline HTML) are **intentional** - do not "fix" them. Fix violations at the source rather than disabling rules.
 2. **Spelling**: All spelling must be clean via the CSpell VS Code integration; words must be correctly spelled in **US English** (the repo-wide convention - see [AGENTS.md](./AGENTS.md)). Project-specific terms go in the workspace CSpell config.
+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.
 
 ## .NET