[docs] Markdownlint cleanup across labs/ READMEs

[ManojRamani]

Problem

Most `README.md` files under `labs/` and the `labs/AGENTS.md` file trip the standard markdownlint ruleset with a mix of pre-existing formatting issues:

• MD022 (blanks-around-headings) — `###` headings followed immediately by a list with no blank line.
• MD032 (blanks-around-lists) — lists not surrounded by blank lines.
• MD040 (fenced-code-language) — code fences without a language tag (mostly the ASCII directory trees).
• MD009 (trailing-spaces) — a few stray trailing spaces.
• MD034 (bare-url) — `https://tavily.com\` / `https://nova.amazon.com/dev-apis\` rendered as bare URLs instead of angle-bracketed or linked.
• MD004 (ul-style) — `labs/module4/README.md` uses `*` bullets while the rest of the repo uses `-`.
• MD060 (table-column-style) — `labs/AGENTS.md` module-overview table pipes aren't space-padded.

None of these affect rendering, but they show up as VS Code / CI lint warnings and add noise whenever someone edits a nearby line.

Affected files

• `labs/AGENTS.md`
• `labs/README.md`
• `labs/module1/README.md` (also has an orphaned trailing empty code fence)
• `labs/module2/README.md`
• `labs/module3/README.md`
• `labs/module4/README.md`
• `labs/module5/README.md`
• `labs/module4/notebooks/mcp_servers/weather_server/README.md` (trivial — missing terminal newline)

Scope

Content is not changing — only whitespace, code-fence language tags, URL bracketing, bullet style, and table pipe padding.

Filing separately from #50 / #51 / #52 because this is pre-existing style drift, not related to any of the model-ID / Chroma work.