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-pwsh/src/index.ts

@@ -138,6 +138,11 @@ const BACKGROUND_OUTPUT_PROPERTIES = {
 /* jscpd:ignore-end */
 
 export function apply(ctx: Context, config: Config = {}): void {
+  // Model commands are written in PowerShell; a mismatched executor would
+  // hand them to bash and surface as ordinary nonzero exits.
+  if (ctx.bash.dialect !== 'powershell') {
+    throw new Error(`tool-pwsh: the mounted executor speaks '${ctx.bash.dialect}', not powershell — mount dsh-pwsh-local or the matching shell tool`)
+  }
   const backgroundEnabled = config.enableRunInBackground ?? true
 
   ctx.systemPrompt.section({