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.

packages/bash/bash-local/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/bash/bash-local/README.md
-README.md: cb8e7f0ae766d9b1c5f1678e77d35992085d3d52
-README.zh.md: 20af9c18998c6f3f0403c50f3a8ac599607dc094
+README.md: b011db6478db8fa4dbdfef4812355d0ed2c1eba8
+README.zh.md: 1ddaf7c283aad40d14764388b5ec1e8ebbdf6d16