Add Antigravity to CI workflows and templates

[thomas-bartlett]

Follow-up to Antigravity format PR. Update CI and templates for full integration.

Tasks

• validate-rules.yml: Add .agent directory check
• build-ide-bundles.yml: Add ide-rules-antigravity.zip to releases
• rule-feedback.yml: Add Antigravity to IDE dropdown
• new-rule.yml: Add Antigravity to formats list