chore(deps): drop unused direct dependency asciidoctor

[DeepDiver1975]

What

Removes the standalone asciidoctor package from dependencies.

Why

asciidoctor is declared as a direct dependency but is never used — there is no require('asciidoctor') / require('@asciidoctor/core') in any source file, and no asciidoctor CLI invocation in scripts, bin/, or CI.

It originally appeared to be needed because asciidoctor-kroki has a peer dependency on @asciidoctor/core >=2.2 <4.0. It turns out it isn't: npm satisfies that peer directly.

Verification

Tested in an isolated worktree with a clean npm install:

	with asciidoctor	without asciidoctor
packages installed	408	343 (−65)
@asciidoctor/core@3.0.4 present	✓	✓ (hoisted for kroki peer)
unmet-peer warnings	none	none
kroki register + convert PlantUML	✓	✓

Antora keeps using its own nested @asciidoctor/core@2.2.9 for rendering; asciidoctor-kroki binds to the hoisted @asciidoctor/core@3.0.4, which npm installs to satisfy the peer dependency even without the top-level asciidoctor package.

Note: a full npm run antora build could not be run in the test environment (Node v18 vs. Antora's undici requiring Node ≥20.18.1 — unrelated to this change). The peer-dependency resolution and runtime kroki conversion are the conclusive signals.

package-lock.json is regenerated accordingly (prunes asciidoctor and its now-orphaned transitives: ejs, nunjucks, pug, @asciidoctor/cli).

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