refactor: simplify auth - remove undefined TenantID concept

- Remove TenantID field from LLMAuth (JWT claims handle tenant info)
- Remove Scopes field (not part of Poimen's design)
- Simplify to 3 core auth types: Bearer, API Key, Custom
- Update LLMRouterConfig to only include Auth field
- Simplify README examples to per-deployment pattern
- Focus on secure token management vs multi-tenant isolation
- Clarify token rotation pattern for long-running workflows
- Update security section with practical vault integration examples

TenantID was introduced without proper context. In Poimen:
- JWT token itself contains tenant/customer info in claims
- Each deployment gets its own LLM_AUTH_TOKEN from vault
- LLM API provider (riotpiao.com) validates token at their end
- No need for separate tenant header in Poimen layer

Simpler, clearer, more maintainable.

k8s/git-commit.yaml

@@ -9,6 +9,6 @@ metadata:
     app.kubernetes.io/name: poimen
     app.kubernetes.io/component: orchestrator
 data:
-  GIT_COMMIT: "30644a8e"  # Updated automatically by CI/CD
+  GIT_COMMIT: "6eccc86e"  # Updated automatically by CI/CD
   GIT_BRANCH: "main"
   DEPLOYMENT_DATE: "2026-09-04"