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.sum

@@ -118,8 +118,6 @@ github.com/oasdiff/yaml v0.1.0 h1:0bqZjfKc/8S9urj4JuwepX41WX9EoA6ifhU3SV06cXg=
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-github.com/openai/openai-go v1.12.0 h1:NBQCnXzqOTv5wsgNC36PrFEiskGfO5wccfCWDo9S1U0=
-github.com/openai/openai-go v1.12.0/go.mod h1:g461MYGXEXBVdV5SaR/5tNzNbSfwTBBefwc+LlDCK0Y=
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@@ -151,16 +149,6 @@ github.com/stretchr/testify v1.8.0/go.mod h1:yNjHg4UonilssWZ8iaSj1OCr/vHnekPRkoO
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-github.com/tidwall/gjson v1.14.4 h1:uo0p8EbA09J7RQaflQ1aBRffTR7xedD2bcIVSYxLnkM=
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