chore: update validation workflow and scripts for README and manifest versions

[birosrichard]

Extend generate_agents.py to check README and manifest versions

agents/AGENTS.md is generated and marketplace.json is validated in CI, but two other hand-maintained surfaces had no guard at all, and both had already drifted:

• gemini-extension.json was on 1.0.0, while plugin.json and marketplace.json were on 2.0.0. Fixed here.
• The README skills table and its Skills-N badge are updated by hand. Nothing stops a new skill from landing without them.

The script already walks these files, so it now checks them too.

Changes

• validate_readme compares the skill names in the ## Skills table against skills/*/SKILL.md in both directions, and checks the badge count (URL and alt text). Only names and counts, not the prose: the table copy is hand-written and better than the SKILL.md descriptions, so it stays human.
• validate_versions — asserts one shared version across plugin.json, gemini-extension.json, marketplace.json metadata, and each per-plugin version in marketplace.json (those are what users actually install).
• All checks now run before exiting, so one CI run reports everything that needs fixing instead of one problem per round.
• Workflow paths now include README.md, plugin.json and gemini-extension.json; otherwise, a PR touching only those files would skip validation entirely.

This is part of the #75

[marekh19]

@claude /code-review