feat(bash): shell tools reject a mismatched executor dialect at load

The seam gains ShellDialect ('bash' | 'powershell' - concrete shells, not
families: zsh or fish would be their own values, never 'bash'); bash-local
declares bash (bash-sandbox inherits), pwsh-local declares powershell, and
both tools throw at load when the mounted executor speaks another dialect -
previously tool-pwsh over bash-local handed PowerShell text to bash -c and
the deployment error surfaced as ordinary nonzero exits. Pinned by mismatch
tests on both tools; the parity note records the contract (both languages).

Also from the review round: the tool-bash README's managed-environment
section becomes a summary linking the owning dsh-bash-env contract (the
duplicated prose carried a stale owner in its example import), the
pwshOnly JSDoc drops the stale 'on PATH' phrasing, and the task-tools
contract comment in the two pwsh compositions is indented into its block.

packages/bash/tool-bash/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/tool-bash/README.md
-README.md: c168b3bfe49faec0be8bd7664411f538a8142edf
-README.zh.md: 3b94f8bc2e4aca8ade15c4e2e1e35d1674c66fdb
+README.md: be89acf06dbc6b4c420dd7fd34eb6ccb842c1fbd
+README.zh.md: 7886addd4c941d81a90546518a5967f64edcdf61