style(antora.yml): satisfy the yamlfmt pre-commit hook

[Bill Traynor (wmat)]

Problem

The pre-commit workflow has been failing on main since 121444f (merge of #253). The yamlfmt hook reformats antora.yml, so the hook reports modified files and the job exits non-zero. The last green run on main was 2aef34e on 2026-07-31.

Because the check runs against the merge with main, every PR opened since then inherits the red check regardless of what it touches — e.g. #256, which does not go near a YAML file the hook rewrites.

Fix

Apply exactly what the hook produces:

+---
 name: common
 title: Common Assets
-version: ~
+version:

Both ~ and an empty value parse to YAML null, so the component stays versionless and Antora's behaviour is unchanged (yaml.safe_load → {'name': 'common', 'title': 'Common Assets', 'version': None} before and after).

Verification

pre-commit run --all-files passes locally with no files modified:

check for broken symlinks............................(no files to check)Skipped
check yaml...............................................................Passed
fix end of files.........................................................Passed
trim trailing whitespace.................................................Passed
Format YAML files........................................................Passed

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