Remove markdownlint exclusion for collections/*.collection.md

[WilliamBerryiii]

Summary

The .markdownlint-cli2.jsonc configuration (line 12) excludes collections/*.collection.md from linting. This exclusion should be removed by fixing the underlying lint violations so these auto-generated files pass the full linting pipeline.

Context

During work on #1256 / PR #1316, the exclusion was investigated for removal. Running markdownlint against these files produces 23 errors across the collection:

Rule	Count	Description
search-replace/no-em-dash	21	Em dash (—) in frontmatter description fields
MD041	1	First line is not a top-level heading
MD013	1	Line length exceeds configured limit

The cleanup was deferred to keep PR #1316 focused on its original scope.

Scope

1. Fix 21 no-em-dash violations — Replace em dashes in frontmatter description fields across collections/*.collection.md files with compliant alternatives (hyphens or rephrased text).
2. Fix MD041 violation — Ensure every collection.md file starts with a top-level heading.
3. Fix MD013 violation — Wrap or shorten the offending long line.
4. Remove the exclusion from .markdownlint-cli2.jsonc line 12.
5. Update references to this exclusion in:
   • scripts/linting/Validate-MarkdownFrontmatter.ps1 (line 36 area)
   • .github/copilot-instructions.md (line 105 area, if still referencing the exclusion)
6. Regenerate plugin outputs — Run npm run plugin:generate after changes to confirm pipeline still passes.

Related

• chore: normalize 'master index' wording and em dash punctuation in collection descriptions #1320 — Covers em dash cleanup in collection bullet descriptions and "master index" wording (different scope)
• collection.md: auto-generation, maturity safety, and custom header support #1256 — Parent feature for collection.md auto-generation and maturity filtering
• PR feat(collections): auto-generate collection.md with maturity filtering #1316 — Implementation PR where this cleanup was identified and deferred

Acceptance Criteria

• npx markdownlint-cli2 "collections/*.collection.md" produces zero errors
• .markdownlint-cli2.jsonc no longer excludes collections/*.collection.md
• npm run plugin:generate and npm run plugin:postprocess pass cleanly
• npm run lint:md passes with no regressions