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.

.agents/notes/implemented/feature/2026-08-02-pwsh-tool-bash-parity.md

@@ -29,6 +29,7 @@ The first Windows-native foundation shipped `dsh-tool-pwsh` as a deliberately mi
 - The bash and pwsh tools are now behaviorally interchangeable for foreground and background shell work (minus sandbox), and the pwsh prompt/description sentences are each backed by the renderer — the reviewer's grep-against-code check passes.
 - Parity ran BOTH ways once: the pwsh tool's structured foreground abort (`HarnessError('tool call aborted', TOOL_ABORTED)` with name `AbortError`) was backported to the bash tool, replacing its uncoded `Error('command aborted')` — a model-visible/logged change pinned by exact-shape tests on both sides and by the cancel-tool-calls fixture.
 - `@deepseek-ai/dsh-bash-env` is a new shipped package; `dsh-tool-bash`'s `dshHome` config moved there, so compositions mounting the shell tools must also mount `bash-env` (the spine bundles do).
+- The seam now carries a `dialect` (`ShellDialect`: `bash` | `powershell` — concrete shells, not families; a POSIX-ish sibling would be its own value), and both shell tools reject a mismatched executor at load, so pairing tool-pwsh with bash-local (or the reverse) fails loud instead of surfacing commands handed to the wrong parser as ordinary nonzero exits.
 - Windows-only semantics (CRLF normalization, forced-termination exit-1/signal-null, POSIX-only self-signal) remain pinned by tests as before.
 - The pwsh tool's per-file coverage gate rides on the scriptable fake-executor suite (`tests/tools.spec.ts`); the real-pwsh integration and Loader-composition suites self-skip where `pwsh` is absent, mirroring the bash suites' division of labor.
 - The roadmap proposal's parity stage is delivered; its remaining stages are the Windows default composition and pwsh TUI/GUI rendering.