Rename to activeadmin_mcp, rewrite README, add RubyGems release

[lloydwatkin]

Summary

• Rewrote the README from scratch to describe how the engine actually works — the list_resources / query (Ransack) / update tools, the ActiveAdmin-respecting update rules, optional devise_token auth, mount strategies, and full config. Credits the original betacraft/active_admin_mcp project; MIT license unchanged.
• Renamed the gem, file/directory paths, generator namespace, and Ruby module from active_admin_mcp / ActiveAdminMcp to activeadmin_mcp / ActiveadminMcp.
• Added RubyGems publishing via GitHub Actions Trusted Publishing (OIDC) — no stored API key:
  • .github/workflows/release.yml triggers when a GitHub Release is published, derives the version from the vX.Y.Z tag, writes it into version.rb, runs specs, publishes via rubygems/configure-rubygems-credentials + gem push, then commits the version bump back to main.
  • Rakefile, CHANGELOG.md, and RELEASING.md (one-time RubyGems setup + release checklist).
  • Gemspec: OLIOEX added as maintainer (harunkumars kept), changelog_uri, rubygems_mfa_required, homepage → OLIOEX/activeadmin_mcp.
• CI trimmed to the latest Ruby (3.4).

Verification

• All 59 specs pass (bundle exec rspec).
• gem build activeadmin_mcp.gemspec produces a correctly-packaged activeadmin_mcp-0.1.0.gem.
• Both workflow YAML files validate.

Follow-ups before the first release

• A pending trusted publisher has been registered on RubyGems for activeadmin_mcp.
• Create a GitHub Environment named rubygems (matches environment: rubygems in the workflow).
• If main has branch protection, allow github-actions[bot] to push the version-bump commit (see RELEASING.md).

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