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/subsystems/bash.md

@@ -266,7 +266,7 @@ abstract run(spec: BashExecSpec): Promise<BashRunResult>
 abstract start(spec: BashExecSpec): BashProcess
 ```
 
-Source: [`packages/bash/bash/src/index.ts:53`](../../packages/bash/bash/src/index.ts)
+Source: [`packages/bash/bash/src/index.ts:65`](../../packages/bash/bash/src/index.ts)
 
 <a id="ctxbashenv--bashenvregistry"></a>