[Phase 1.3] Temporal configuration + secrets (#11)

## Changes
- `internal/config/config.go` — Env-specific loading (dev/staging/prod), validation, in-cluster detection
- `internal/config/config_test.go` — 11 tests covering defaults, env override, TLS, prod gates
- `k8s/secrets.enc.yaml` — SOPS-encrypted Secret (anthropic key, memory JWT, temporal postgres pw)

## Validation
- 11 config tests pass
- Full test suite passes (`go test ./...`)
- `go build ./...` clean
- SOPS encryption verified (age key)

k8s/secrets.enc.yaml

@@ -0,0 +1,25 @@
+apiVersion: ENC[AES256_GCM,data:9JQ=,iv:ugaPXZZ0mwj9ub3AOBbevh3Eej0ik9IRGh6my37euxk=,tag:4TNha8uQeB9RP5sFWZCEug==,type:str]
+kind: ENC[AES256_GCM,data:Sb+P4zNR,iv:pwzIwcXjgKfCFPi63E77QE2zaFFuthtMNLNU+CvXoJQ=,tag:xg48gnnKBGWbJWEnTm9T9w==,type:str]
+metadata:
+    name: ENC[AES256_GCM,data:7T4kCUDf0RaWwitBPaE=,iv:tS7l6FSejcYl7MobbBtVmVn0CBFTCx0BaMkPILJy49s=,tag:huZp3TnSPK6dOtWjmrssSA==,type:str]
+    namespace: ENC[AES256_GCM,data:WWuEZ7Ro,iv:c00ZiQgABdg9Rs0VibYaOSWZ/k2ErDb/dELLjABx8yA=,tag:3ltoqvNuZilxGtdGhNftJg==,type:str]
+type: ENC[AES256_GCM,data:hYyckkSD,iv:0VXD2fV21xgVKxYeZ8hetgpqLpwz5e9yyrImTiYj6w8=,tag:JJtmFuY4rtv77ZyqwEIsmw==,type:str]
+stringData:
+    anthropic-api-key: ENC[AES256_GCM,data:1SMZxO2HcLCmXkTVfvl50pPyRqWDKw==,iv:t4AD4rM7th1fcQJcY4SflV1xTMoVjYjq1zmduKlCkjA=,tag:vhAVGG9jq7c5TqOxEx1sKg==,type:str]
+    memory-service-jwt: ENC[AES256_GCM,data:hqW1u5OROqPlEX4DhoMWCzK0Mw==,iv:/SdcHGNm3yTpPFZI648kVKQT4TDVJ2hbc807QLUvlx4=,tag:KTWn/DQ7qE4wdsHR+Giefw==,type:str]
+    temporal-postgres-password: ENC[AES256_GCM,data:WvyP8a28+Q1u7DRPHN9mPdfoVkl4a0nUSYM=,iv:tASr/phQdN/VoG0u6NDClOBhmb9kJvvhrWo+06oNQnQ=,tag:DyAT8E0CiyxiPnnmJ/wsYQ==,type:str]
+sops:
+    age:
+        - enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSBUcVR6V3hsL1BaMUJrNVpV
+            cThZdVg5RFNhYjlUZkVoMkQwYURYd2dhWVRJCkFWbm5FVHVKOE9pdlg1TUlXMDl3
+            UlBsODF5eU5PamFXU3BoMzZoTFNSQ2MKLS0tIGtSQm54S3dqSDJzYUVpNTd1bkI1
+            Z2dZZ1FDU0tRN1JvVURSMHNua1U2L1kKjFGbdNJxguRYJe5ral3BsFTbopfkvrQC
+            8DCMLl9GaRlyh2k0jJab7/0iCzcLNfOwZJRZHVXA5EjtC0fQLxRqgA==
+            -----END AGE ENCRYPTED FILE-----
+          recipient: age1e5fq3hwxy78psus2nfvmtmua36g0u3suk78ephw6246l974d2utsvn0hla
+    lastmodified: "2026-09-08T23:32:05Z"
+    mac: ENC[AES256_GCM,data:rKsgwD3eTfMTZWXZxmSNfj8A/yAvfc+uC/7XrWU1yMjUxj4/V9MovvKGGhR8KCjFTuYcu0x9JdTtET5QtuOXo//Ly08mwhfqaOX09Fn09V906O+Sx4e+zCNwQItz6VE+yqTRiSepKzE8DQhFmYFwuY/QMXrP1BLHpvm0kkhnFaU=,iv:h3AtYk0hqoFCj+rTmMbM4+a4WMXdMIgW94ysXZ3eJZ0=,tag:q0hLai5tTGNR7/2xdUHkug==,type:str]
+    unencrypted_suffix: _unencrypted
+    version: 3.13.2