Add GitHub Actions CI running mvn verify on JDK 27

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G71TQ5jDETVwdUJxGAwaiT

docs/PLAN.md

@@ -173,7 +173,7 @@ Actuator is optional; when the `pgpkeys` endpoint is exposed
 Extension points: `KeyDiscoveryCustomizer` (wraps WKD discovery below the cache) and `SecureMailSenderCustomizer`.
 
 ## Milestones
-1. **Skeleton** — parent POM, modules, CI. ✅
+1. **Skeleton** — parent POM, modules, CI (GitHub Actions, `mvn verify` on Temurin 27). ✅
 2. **WKD discovery + cache** — `WkdKeyDiscovery`, `HttpWkdFetcher`, `CachingKeyDiscovery`, static/composite discovery,
    properties + autoconfig for discovery beans. Tests: z-base-32 vector, advanced/direct fallback matrix,
    size limit, no redirects, cache TTLs. ✅