feat(overlay, rhdh-local): add branch-based metadata lookup

[dzemanov]

Summary

• updates fetch-plugin-metadata.py script to lookup plugin metadata for branch, default main (to answer questions about what plugin tag or config to use for rhdh release 1.9 and similar)
• fetch-plugin-metadata.py now returns also package version
• adds dynamicArtifact and version to package's metadata-format.md
• adds metadata lookup information to overlay skill
• fixes outdated link to marketplace plugin -> renamed to extensions

[durandom]

Review

Good feature — branch-based metadata lookup is the missing piece for answering "what plugin version ships in RHDH 1.9?" without guessing.

Code review

The Python changes are clean:

• _contents_url() and _raw_url() correctly use urllib.parse.quote(branch, safe="") to handle branch names with special chars (e.g., release/1.9)
• --branch parameter is threaded through all call sites consistently (list_plugins, _list_workspace_metadata, fetch_plugin_metadata)
• version field added to output alongside dynamicArtifact — makes the metadata actually useful for version queries
• Human-readable output updated to show branch and version

Issues

1. Version bump conflict. This PR bumps 0.5.0 → 0.5.1, but the current main may already be at a higher version if any of PRs #49/#58/#54 merged first. You'll need to rebase and adjust the version to whatever main is at + patch.

2. metadata-format.md update is good. Adding dynamicArtifact and version to the documented spec captures what was already present in real metadata files but undocumented.

3. Overlay SKILL.md — the new menu item 5 renumbers 5→6, 6→7, 7→8. The routing table is updated to match. Clean.

4. Documentation link fix — marketplace/README.md → extensions/README.md catches a real rename. Good catch.

Recommendation

Merge-ready after a version rebase. The code changes are solid.