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.

tsconfig.json

@@ -33,6 +33,7 @@
     { "path": "./packages/core/agent-loop" },
     { "path": "./packages/core/agent-core" },
     { "path": "./packages/bash/bash" },
+    { "path": "./packages/code-runtime/code-runtime" },
     { "path": "./packages/llm/llm-deepseek" },
     { "path": "./packages/llm/llm-pi-ai" },
     { "path": "./packages/bash/bash-local" },