feat(poimen): pi sandbox workspace volume, encrypted api keys, cluster dns rewrite (#54)

Adds the pi coding-agent sandbox's cluster side:
- `k8s/infra/poimen/pi-agent-workspace-pv.yaml` — static hostPath PV + PVC for the sandbox /workspace (Longhorn cannot attach new volumes in this cluster; this gives real persistence and survived a pod delete/recreate test: marker file + 38 KB session.jsonl intact, the session answered from its own history afterwards).
- `k8s/argocd/secrets/pi-sandbox-secrets.enc.yaml` — the sandbox's API-key Secret, SOPS-encrypted (recipient from .sops.yaml, payload only: `encrypted_regex ^(data|stringData)$`), registered in the ksops generator. Round-trip verified against the live Secret.
- `terraform/files/coredns/Corefile` — `rewrite name pi.riotpiao.com ingress-nginx-controller...` so the hostname resolves in-cluster.

Note: the keys currently in that Secret are the ones leaked earlier and must be rotated; re-encrypt with `sops -e -i` after rotating.Reviewed-on: #54
Co-authored-by: rock <[email protected]>

k8s/argocd/secrets/pi-sandbox-secrets.enc.yaml

@@ -0,0 +1,24 @@
+apiVersion: v1
+kind: Secret
+type: Opaque
+metadata:
+    name: pi-code-agent-api-keys
+    namespace: poimen-sandbox
+stringData:
+    ANTHROPIC_API_KEY: ENC[AES256_GCM,data:TBcno/dzj3qykHjeX96UKZekuA/WMQT2E7a1ziPyYo5J6lWLPydgli84KeJZhH62/8CnnFtFOhVIe4FWCGPGjvqKH6M8WbiQHaP9y7FN0qQUpFHKNVAQ2R+s0wInhw0eUmANvd7bRx8aIZ2X,iv:oex19yVtWUevoJ0A3mkMLCfWu85o1gPSNp43x1iV0LE=,tag:BW3zQnwvagPceeNird9CZg==,type:str]
+    DEEPSEEK_API_KEY: ENC[AES256_GCM,data:vq/5gOsMVjlVMd0oXiBtHFHFEr+IF2YR2iP2OBFWx3ZFNoQ=,iv:QhLU/BRIJb/7QKDCRyw+HWPphZifVEhIOLywYDgOb20=,tag:BZdtXw+WsuK+P4E0caTa0Q==,type:str]
+sops:
+    age:
+        - enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSBqNm1LNE5nWktnd3c3STg3
+            UlVWbWJxWElxUHMwa0JVMk81Z0c4d0JaQm1vCnpLQWJWV3hxOU00NzJIaG51Mmlr
+            ZWpNTFJWcmdtV0w4YndQeHpvSUx2MTAKLS0tIDVnNkNuVWhPUFZ5V2hFOEt1eitG
+            blVqRy8yNnYzSU1FNzZGNkxrRElvWVEKL3J77INje75+CvKXaJHC3CvONgHv7d0a
+            a10MITvfreFr6M4AN3Z/mno49SDMp15Q4oQRagHHxSTS1K54mWHo8g==
+            -----END AGE ENCRYPTED FILE-----
+          recipient: age1e5fq3hwxy78psus2nfvmtmua36g0u3suk78ephw6246l974d2utsvn0hla
+    encrypted_regex: ^(data|stringData)$
+    lastmodified: "2026-10-06T01:19:49Z"
+    mac: ENC[AES256_GCM,data:GpYl8IRefms36GvO3RW41c0US7SOgoOhd4rumxsCdEf5ME9kJ0eOxPx8klMgnlYrkBnwHmad8jj7C+HcA5Eim7mdUaYOZZWPFohvC6N4GLHX85aH78P09CelqG5tdm/Gm+1QfxYu0vANYGCAVjLK7YbBbZjbZmbvVfQvAbs3mOY=,iv:y3lglGu8FhTHQpO1poSIKgGYJFu5J9yjIaOjulda6fY=,tag:5Sgh4TbtNFMw7xOJjYRn0g==,type:str]
+    version: 3.13.2