okf: write stackoverflow bundle tags as YAML lists

[amirhormati]

Problem

tags: in OKF frontmatter is meant to be a list. Eight files in
okf/bundles/stackoverflow write it as a bare comma-separated string instead.

A JavaScript string is iterable, so the Documents Layout iterates it anyway and
each character becomes a Dataplex label. Pushing tables/votes.md
(tags: Stack Overflow, votes, posts, community) to Knowledge Catalog stored:

{" ": "true", ",": "true", "O": "true", "S": "true", "a": "true", "c": "true", ...}

No error, no warning. Those eight files round-tripped corrupted; the other 24
in the bundle were fine.

Fix

Rewrite the eight files as YAML flow sequences, matching the style the nine
acme_retail files already use:

tags: [Stack Overflow, votes, posts, community]

The brackets are the whole difference. Without them it is a plain scalar, not a
sequence.

Files changed:

datasets/stackoverflow.md, tables/posts_answers.md,
tables/posts_moderator_nomination.md, tables/posts_questions.md,
tables/posts_wiki_placeholder.md, tables/stackoverflow_posts.md,
tables/users.md, tables/votes.md

Bundle data only. #294 adds the library-side guard that makes this shape an
error instead of silent corruption; it should land after this PR, since these
are the files that trip it.

Verification

Against hormati-bqml / us-central1, using the demo's push/pull path:

• Before: 24 of 32 stackoverflow files round-tripped losslessly.
• After: 32 of 32.
• tables/votes now stores four labels (Stack Overflow, votes, posts,
  community) instead of fourteen single characters.

Found while round-tripping the sample bundles for #292. #292, #293, and #294
share no files.

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