ci: enable Grok qualification in the trusted paid workflow (#13845)

## Thinking Path

> - Paperclip manages AI agents and their work.
> - Product E2E tests verify tasks through the browser, server, and
runner.
> - These paid tests use a trusted workflow from master and an isolated
target commit.
> - The Grok target branch selects XAI_API_KEY, but the trusted workflow
does not deliver that credential.
> - Its artifact test also needs the pinned Python verifier before
execution.
> - This pull request adds both bindings inside the existing paid
boundary.
> - The tests can then run on the configured EC2 fleet without laptop
Docker.

## Linked Issues or Issue Description

Related evaluation infrastructure:
https://github.com/paperclipai/paperclip/pull/11297. No duplicate Grok
paid-workflow change was found.

**What existing behavior does this improve?**

Branch-targeted Grok Product E2E qualification in the existing paid
workflow.

**Current behavior**

Grok cells cannot receive their selected API credential. The Grok
build-revise case also misses the artifact-verifier setup step.

**Proposed behavior**

Deliver XAI_API_KEY only when the selected matrix credential is
XAI_API_KEY. Prepare the existing pinned verifier for the Grok
qualification suite.

**Reason and benefit**

Run the controller, browser, runner and artifact checks on the EC2
fleet. Preserve default-branch workflow authorization and protected
environment secret access.

## What Changed

- Bind the selected XAI credential only in the paid test step.
- Install the checksum-verified Grok binary for local cells before
provider access.
- Include Grok qualification in the existing pinned artifact-verifier
preparation.
- Add an optional max_parallel input that can only lower the configured
campaign concurrency. Use 1 for the Grok test key.
- Extend security assertions and document setup.

## Verification

- Ran the Product E2E workflow-security tests: 11 passed.
- Checked the diff for whitespace errors.
- Reviewed credential selection, setup ordering, numeric actor gates,
target commit pinning, and trusted report checkout.
- Live Grok execution follows after this workflow is available on
master. This PR does not claim completed Grok qualification.

## Risks

The paid test step can use the selected XAI credential and incur
provider charges. The credential remains in runner-e2e-paid and is
absent from setup, build, and reporting jobs. The default-branch gate
and existing environment restrictions remain in place. No database
migration or product behavior changes.

## Model Used

OpenAI Codex, GPT-6 family, with reasoning, code editing, and tool
execution. The exact deployment model ID and context-window size are not
exposed in this session.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [x] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [ ] All Paperclip CI gates are green
- [ ] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

tests/runner-e2e/SECURITY.md

@@ -1,6 +1,6 @@
 # Runner E2E security for a public repository
 
-This suite can spend provider money, expose four API credentials to isolated
+This suite can spend provider money, expose selected API credentials to isolated
 test processes, publish a container, retain private visual evidence, and write
 public structured evidence. Treat changes to the workflow, harness, fixture
 prompts, evidence packager, and publisher as security-sensitive production
@@ -64,7 +64,7 @@ rejects mutable tag or branch references.
 ## Secrets and protected environments
 
 Create `runner-e2e-paid`, restrict deployments to the default branch, and put
-only `OPENAI_API_KEY`, `ANTHROPIC_API_KEY`, `OPENROUTER_API_KEY`, and
+only `OPENAI_API_KEY`, `ANTHROPIC_API_KEY`, `OPENROUTER_API_KEY`, `XAI_API_KEY`, and
 `DAYTONA_API_KEY` in it. Do not duplicate these credentials as repository- or
 organization-level Actions secrets: environment scoping is the boundary that
 prevents branch or pull-request jobs from requesting them. Require approval