ci: add advisory CSpell check

[marcleblanc2]

Summary

• add CSpell 9 as a pnpm dev dependency and expose pnpm spellcheck for full-repository local checks, including dotfiles
• add a checked-in Sourcegraph/project dictionary and explicitly flag likely legacy misspellings
• on pull requests, report only spelling errors on lines added by the PR, so existing repository findings are ignored
• create or update one PR comment containing grouped file, line, column, word, and context details; resolve the comment after fixes
• keep the PR check advisory while preserving the local command's failing exit code
• document that CSpell is only installed by pnpm and does not run automatically during normal development

Fork PRs have a read-only token, so their report appears in the workflow log without a PR comment.

Verification

• simulated an added typo: the diff checker exited 1 and reported exactly that word, line, column, and context
• reverted the typo: the diff checker exited 0 despite pre-existing repository findings
• Prettier checks passed for the script, package/config, and workflow
• workflow YAML parsed successfully and every shell run block passed bash -n
• node --check dev/check-spelling.mjs
• git diff --check

Exclusions

CSpell honors .gitignore, so dependency, cache, and build output remain excluded. The full local check additionally skips binary image/font formats, pnpm-lock.yaml, CSpell's own config/dictionary, and checked-in generated outputs (docs/technical-changelog.mdx, generated observability alert/dashboard references, and the generated AWS icon module).

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Project	Deployment	Actions	Updated
sourcegraph-docs	Ready	Preview	Sep 6, 2026 10:36am UTC