revert(bash): drop the executor dialect guard

The ShellDialect marker on BashExecutor and the load-time rejection in
tool-bash/tool-pwsh force every test and example fake executor to declare
a dialect for a guard with no in-repo or plausible deployment to catch
(shipped compositions always pair the tool with its matching executor),
and they break the example composition suites that stub ctx.bash without
a dialect (agent-spine-demo/cli-demo/acp-demo lost the bash tool).

Keep the non-dialect doc fixes that landed with the attempt: the tool-bash
README bashEnv contract trim with the stale example import removed, the
acp-snapshot suite.ts 'usable pwsh' JSDoc, and the pwsh.cordis.yml comment
indent. The parity note records the attempt and revert under Alternatives.

packages/context/tmux-context/tests/tmux-context.spec.ts

@@ -48,8 +48,6 @@ function runResult(stdout: string, overrides: Partial<BashRunResult> = {}): Bash
 
 /** A scriptable fake `ctx.bash` recording the command it was asked to run. */
 class FakeBash extends BashExecutor {
-  readonly dialect = 'bash' as const
-
   commands: string[] = []
   result: BashRunResult = runResult(`${tmuxLine()}\n`)
   runError?: Error