Turn Markdown lint back on

[mpaulosky]

Markdown lint enforces nothing in this repo today.

• .markdownlint.json is { "default": false }, which turns off every rule.
• The Lint Markdown workflow passes it as its config (config: ".markdownlint.json"), so the markdownlint check always passes.
• markdownlint-cli2 also reads .markdownlint.json when it runs from scripts/gate.sh and the pre-commit hook. The rules in .markdownlint-cli2.jsonc (line length 200; MD033, MD036, MD041 and MD060 off) therefore never apply either.

Size of the gap

Linting with .markdownlint-cli2.jsonc alone, using the workflow's globs, finds 714 violations in 64 files. Most are formatting that markdownlint-cli2 --fix rewrites on its own:

Rule	Count
MD022 blanks around headings	252
MD032 blanks around lists	197
MD031 blanks around fences	166
MD034 bare URLs	127
MD013 line length	50
MD012 multiple blank lines	40
MD040 fence language	32
Others (MD026, MD047, MD025, MD029, MD024, ...)	fewer than 25 each

The heaviest files are the test guides under docs/guides/, the v0.0.9 release post and docs/reviews/squad-ci-review-issue-17.md.

Proposed fix

1. Run markdownlint-cli2 --fix over the workflow's globs, then fix the rest by hand (or relax a rule in .markdownlint-cli2.jsonc where the repo's style disagrees).
2. Delete .markdownlint.json, and point the workflow at .markdownlint-cli2.jsonc (or drop its config: input, since markdownlint-cli2 finds that file itself).
3. Exclude docs/blogs/** if generated release posts shouldn't be held to the rules, as IssueTracker does.

The CI, gate.sh and pre-commit hook then all enforce the same rules. Found while porting the pre-commit hook for #227.