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/support/acp-snapshot/src/suite.ts

@@ -162,7 +162,7 @@ export interface Scenario {
    */
   posixOnly?: boolean
   /**
-   * Whether the scenario boots a composition that needs a real `pwsh` on PATH
+   * Whether the scenario boots a composition that needs a usable `pwsh`
    * (the pwsh-tool-turn scenario). The run test is skipped when the suite's
    * {@link SnapshotSuiteOptions.hasPwsh} probe is false; fixtures stay guarded
    * on every platform.