docs(bash): reframe stdin/env — the scrub is the security control, not a trust boundary

Address review: the "trusted-plugin surface" framing overstated the security
story. A model driving the `bash` tool already has equivalent power to set env
vars and feed stdin through ordinary shell syntax (`FOO=bar cmd`, heredocs), so
the `env`/`stdin` seam fields grant it no new capability — and they cannot
exfiltrate the harness's ambient credentials, because the credential SCRUB in
dsh-bash-local (which strips *KEY*/*SECRET*/*TOKEN* from process.env before the
child sees it) is the actual control, and it works regardless of these fields
(tool-call args are static JSON, never shell-evaluated).

So drop the "dangerous / trusted-plugin boundary" language across the RFC, the
three bash-package READMEs, the bash/src/types.ts JSDoc, and docs/bash.md (both
the type-equiv blocks — kept 1:1 with source — and the prose). The reality that
remains: the `bash` tool doesn't EXPOSE env/stdin as parameters because they'd
be redundant with shell syntax; the fields exist for in-process plugins (the
hooks bridges) to pass a JSON payload + CLAUDE_* vars cleanly. The guard test is
kept but reframed: it catches a future `...args` spread that would silently
forward model input into the post-scrub env merge, NOT a trust wall. No code or
behavior change.

packages/bash/tool-bash/tests/tools.spec.ts

@@ -864,14 +864,18 @@ describe('tool-owned UI presentation (presentCall / presentResult)', () => {
   })
 })
 
-describe('trusted-plugin boundary: the model-facing bash tool never sets env/stdin', () => {
+describe('the model-facing bash tool builds its request from named args only (no {...args} forward)', () => {
   /**
    * Records every {@link BashExecRequest} the consumer hands to `resolve()`, so a
-   * test can assert what the model-facing tool DID and DID NOT forward. `stdin`
-   * and `env` are a TRUSTED-PLUGIN surface (in-process plugins only); the `bash`
-   * tool must never thread model-supplied input into them, even when the model
-   * smuggles extra keys into the tool arguments. Foreground `run()` returns a
-   * canned result; `start()` is unused here.
+   * test can assert what the model-facing tool DID and DID NOT forward. The `bash`
+   * tool does not expose `stdin`/`env` as parameters (bash syntax already gives a
+   * model that power), so it must build its request from named args only and
+   * never spread unknown tool-call keys into it. This guard's job is to catch a
+   * future refactor that blindly forwards `...args` — which would silently thread
+   * model input into the post-scrub `env` merge — NOT to defend a trust boundary
+   * (the credential scrub in dsh-bash-local is the security control; see the
+   * bash-stdin-env RFC). Foreground `run()` returns a canned result; `start()` is
+   * unused here.
    */
   class RecordingBashExecutor extends BashExecutor {
     readonly requests: BashExecRequest[] = []
@@ -911,12 +915,14 @@ describe('trusted-plugin boundary: the model-facing bash tool never sets env/std
     return { ctx, bash: ctx.bash as RecordingBashExecutor }
   }
 
-  it('does not forward env/stdin even when the model smuggles them as extra arguments', async () => {
+  it('does not forward env/stdin even when the model includes them as extra arguments', async () => {
     const { ctx, bash } = await setupRecording()
-    // Adversarial args: the model includes `env` and `stdin` keys (and a
-    // credential-shaped value) hoping they reach the executor. The bash tool's
-    // schema ignores unknown keys, and execute() builds the request from only
-    // command/workdir/timeoutMs/signal — so the recorded request carries NEITHER.
+    // Extra args: the model includes `env` and `stdin` keys hoping they reach the
+    // executor. The bash tool's schema ignores unknown keys, and execute() builds
+    // the request from only command/workdir/timeoutMs/signal — so the recorded
+    // request carries NEITHER. (Not a security wall — the model could set an env
+    // var or feed stdin via shell syntax anyway; this just keeps the request
+    // shape honest so a future `...args` spread can't silently forward input.)
     await ctx.tools.execute({
       callId: CallId('boundary-1'),
       name: 'bash',