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

@@ -48,6 +48,8 @@ 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