feat(phase-1.3): config validation, env-specific loading, SOPS secrets

config.go:
  - Environment enum (dev/staging/prod) with auto-detect
  - MemoryServiceConfig + LLMConfig structs
  - Validate(): TLS pair check, file existence, prod LLM key requirement
  - In-cluster default host detection via KUBERNETES_SERVICE_HOST
  - TaskQueue + WorkerCount from env

config_test.go: 11 tests
  - defaults, env override, TLS mismatch, TLS file not found
  - prod validation (requires LLM key), env parsing
  - in-cluster detection, port defaults, int parsing

k8s/secrets.enc.yaml: SOPS-encrypted Secret
  - anthropic-api-key, memory-service-jwt, temporal-postgres-password
  - Age-encrypted, KSOPS-compatible

Build: clean, full test suite passes

.gitignore

@@ -7,3 +7,7 @@
 starter
 worker
 poimen
+
+# Compiled binaries
+poimen-worker
+poimen-api