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.

vendor/modules.txt

@@ -270,28 +270,6 @@ github.com/oasdiff/yaml
 # github.com/oasdiff/yaml3 v0.0.13
 ## explicit; go 1.22.5
 github.com/oasdiff/yaml3
-# github.com/openai/openai-go v1.12.0
-## explicit; go 1.21
-github.com/openai/openai-go
-github.com/openai/openai-go/internal
-github.com/openai/openai-go/internal/apierror
-github.com/openai/openai-go/internal/apiform
-github.com/openai/openai-go/internal/apijson
-github.com/openai/openai-go/internal/apiquery
-github.com/openai/openai-go/internal/encoding/json
-github.com/openai/openai-go/internal/encoding/json/sentinel
-github.com/openai/openai-go/internal/encoding/json/shims
-github.com/openai/openai-go/internal/paramutil
-github.com/openai/openai-go/internal/requestconfig
-github.com/openai/openai-go/option
-github.com/openai/openai-go/packages/pagination
-github.com/openai/openai-go/packages/param
-github.com/openai/openai-go/packages/respjson
-github.com/openai/openai-go/packages/ssestream
-github.com/openai/openai-go/responses
-github.com/openai/openai-go/shared
-github.com/openai/openai-go/shared/constant
-github.com/openai/openai-go/webhooks
 # github.com/oracle/oci-go-sdk/v65 v65.95.2
 ## explicit; go 1.13
 github.com/oracle/oci-go-sdk/v65/common
@@ -326,18 +304,6 @@ github.com/segmentio/encoding/json
 # github.com/sony/gobreaker v0.5.0
 ## explicit; go 1.12
 github.com/sony/gobreaker
-# github.com/tidwall/gjson v1.14.4
-## explicit; go 1.12
-github.com/tidwall/gjson
-# github.com/tidwall/match v1.1.1
-## explicit; go 1.15
-github.com/tidwall/match
-# github.com/tidwall/pretty v1.2.1
-## explicit; go 1.16
-github.com/tidwall/pretty
-# github.com/tidwall/sjson v1.2.5
-## explicit; go 1.14
-github.com/tidwall/sjson
 # github.com/yosida95/uritemplate/v3 v3.0.2
 ## explicit; go 1.14
 github.com/yosida95/uritemplate/v3