Scope CI spell-check to README + HISTORY (main-only) (#515)

Cherry-picks the CI cspell-scope fix onto main, without dragging the
other in-flight work currently on develop.

This narrows the CI spell-check on **main** from all markdown
(`**/*.md`) **to** README.md + HISTORY.md, matching template PR
ptr727/ProjectTemplate#302. markdownlint stays repo-wide. The same
change is already on develop; this is the surgical main-only path so the
feature/dependency work on develop is not promoted with it.

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

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ptr727

.github/workflows/validate-task.yml

@@ -51,7 +51,9 @@ jobs:
       - name: Spell check step
         uses: streetsidesoftware/cspell-action@de2a73e963e7443969755b648a1008f77033c5b2 # v8.4.0
         with:
-          files: '**/*.md'
+          files: |
+            README.md
+            HISTORY.md
           incremental_files_only: false
 
       - name: Lint workflows step