feat: add the code-execution capability seam (ctx.codeRuntime)

New group packages/code-runtime/ with the interface package
@deepseek-ai/dsh-code-runtime, per the Code Mode RFC: abstract CodeRuntime
service (run() resolves program failures as an error field, rejects only
for seam misuse), the CodeRunRequest/CodeBindingNamespace/CodeRunResult/
CodeLogEntry/CodeRunFailure vocabulary, and readonly language/isolation
backend descriptors. Registered in the tsconfig maps, packages/README,
architecture service map, and the doc-graph service-role classification;
catalogs regenerated.

The RFC's one forward path token to the worker package becomes an npm-name
mention until PR3 creates that directory (verify-package-paths is
drift-scoped: the now-existing group made the token checkable).

docs/architecture.md ceiling 1630 -> 1640: the doc gained a genuinely new
capability-service row; the row itself is already minimal.

docs/module-graph.md

@@ -78,6 +78,9 @@ flowchart TD
     pkg_app_boot["app-boot"]
     pkg_stdio_agent["stdio-agent"]
   end
+  subgraph group_code_runtime["packages/code-runtime"]
+    pkg_code_runtime["code-runtime"]
+  end
   pkg_llm --> pkg_brand
   pkg_bash --> pkg_brand
   pkg_llm_deepseek --> pkg_llm
@@ -208,6 +211,7 @@ flowchart TD
 | --- | --- | --- |
 | [`brand`](../packages/util/brand) | `util` | — |
 | [`app-boot`](../packages/ui/app-boot) | `ui` | — |
+| [`code-runtime`](../packages/code-runtime/code-runtime) | `code-runtime` | — |
 | [`llm`](../packages/llm/llm) | `llm` | [`brand`](../packages/util/brand) |
 | [`bash`](../packages/bash/bash) | `bash` | [`brand`](../packages/util/brand) |
 | [`llm-deepseek`](../packages/llm/llm-deepseek) | `llm` | [`llm`](../packages/llm/llm) |