feat: decommission nomos agent runtime (plan §5)

Replace nomos with dsh (DeepSeek Harness) as the agent runtime per
plans/2026-08-16-dsh-as-agent-replace-nomos.md section 5.

Deleted:
- cmd/nomos/ (agent loop, :8092 chat gateway, task tools, eval runner)
- nomos/ (SOUL.md, config.yaml, skills/)
- internal/nomos/{session,messagequeue,retrycap,turngate,assent}/
- compose/nomos/ Dockerfile + docker-compose nomos service
- httpapi /agent reverse-proxy mount (NOMOS_PROXY_URL)
- evals/*.yaml (nomos golden-conversation manifests)
- compose/caddy/Caddyfile.oikos nomos.hubris.network block + /agent path
- .golangci.yml nomos-isolation rules
- go.mod openai-go dependency (go mod tidy)

Kept:
- internal/adapters/postgres assent-window logic (governance port)
- OIKOS_NOMOS_AGENT_SLUG config (MCP agent entity attribution)
- Full REST API, MCP tools, policy, scheduler, secrets

Cleaned: deploy.sh image prune, verify-phase6.sh checks,
seed-secrets.sh key source, README/CONTRIBUTING/AGENTS.md docs.

Build, vet, test, compose config all pass.

go.mod

@@ -13,7 +13,6 @@ require (
 	github.com/jackc/pgx/v5 v5.10.0
 	github.com/modelcontextprotocol/go-sdk v1.6.1
 	github.com/oapi-codegen/runtime v1.4.2
-	github.com/openai/openai-go v1.12.0
 	golang.org/x/crypto v0.53.0
 	golang.org/x/sync v0.21.0
 	golang.org/x/sys v0.46.0
@@ -65,10 +64,6 @@ require (
 	github.com/segmentio/asm v1.1.3 // indirect
 	github.com/segmentio/encoding v0.5.4 // indirect
 	github.com/sony/gobreaker v0.5.0 // indirect
-	github.com/tidwall/gjson v1.14.4 // indirect
-	github.com/tidwall/match v1.1.1 // indirect
-	github.com/tidwall/pretty v1.2.1 // indirect
-	github.com/tidwall/sjson v1.2.5 // indirect
 	github.com/yosida95/uritemplate/v3 v3.0.2 // indirect
 	github.com/youmark/pkcs8 v0.0.0-20240726163527-a2c0da244d78 // indirect
 	go.opentelemetry.io/auto/sdk v1.2.1 // indirect