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/module-graph.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 docs/module-graph.md
-module-graph.md: e1c52b700a45e24c78a505bb522f0b3259415187
-module-graph.zh.md: 26306588773d715a88b901edba5391abff19211d
+module-graph.md: 65741283b5a48817bd01c42fe7982fff6c89511a
+module-graph.zh.md: 00ea2925c6636a65e224c899ee264a4e87d4a2cd