[MegaLinter][valid] Markdown polish: markdownlint + markdown-table-formatter findings

From the 2026-07-24 Analyze run. Classified as valid findings (bucket 1), all low-severity docs polish.

markdownlint (84 findings, by rule)

• MD040 (fenced code block missing language) — by far the most common, across most demos/*/README.md files, CONTRIBUTING.md, .github/skills/*.md. Easy bulk fix, just needs a human to pick the right language tag per block (mostly bash/text/json from context).
• MD013 (line length, limit 400) — README.md (10 occurrences) and demos/injection_deep_gradle/README.md (6), all on long inline example/config lines rather than prose. Given these docs use wide inline tables/examples, you may want to raise or disable the line-length limit for markdown specifically rather than reflow these lines — your call.
• MD058 (tables need surrounding blank lines) — README.md, 4 occurrences, on the semantic-conventions tables.
• MD041 (first line should be a top-level heading) — README.md, demos/README.md, .github/constant-instructions.md. Note README.md opens with an italic aside before its H1 (_(This repository is also available under the aliases ...)_), which reads like a deliberate pattern rather than an oversight — might be worth a per-file exception (front_matter_title/first_line_heading config) rather than moving that line.
• MD034 (bare URL) — .github/workflows/monitor_changelog.md:33, 2 occurrences.
• MD010 (hard tabs) — demos/injection_deep_python/README.md, 8 occurrences.
• MD060 (table column spacing style) — .github/skills/ci-troubleshooting.md, 6 occurrences.
• MD009 (trailing spaces) — README.md:351, 1 occurrence.

markdown-table-formatter

2 files need table reformatting: .github/skills/ci-troubleshooting.md, README.md — consistent with the MD058/MD060 findings above.