feat(bash): resolve executor config through the bash settings namespace

The capability's namespace is owned by the seam because it names the
capability, not an implementation: a host composes exactly one provider of
ctx.bash, so both executor families register the same namespace with their
own schema and composition entry without ever colliding, and a settings
document carried between platforms keeps resolving on both.

Both executors read their config through a source thunk, so a stored change
reaches the next command. The constructor checks the schema cannot express
become the section validator, refusing a bad value at the write instead of
at the next command. pwsh re-resolves its executable only when the declared
path changed, so an unrelated settings change never re-probes the filesystem.

docs/config-catalog.md

@@ -315,7 +315,7 @@ export interface Config {
 }
 ```
 
-Source: [`packages/bash/bash-local/src/index.ts:40`](../packages/bash/bash-local/src/index.ts)
+Source: [`packages/bash/bash-local/src/index.ts:41`](../packages/bash/bash-local/src/index.ts)
 
 ## `@deepseek-ai/dsh-bash-sandbox`
 
@@ -1237,7 +1237,7 @@ export interface Config {
 }
 ```
 
-Source: [`packages/bash/pwsh-local/src/index.ts:54`](../packages/bash/pwsh-local/src/index.ts)
+Source: [`packages/bash/pwsh-local/src/index.ts:55`](../packages/bash/pwsh-local/src/index.ts)
 
 ## `@deepseek-ai/dsh-pwsh-sandbox`