ci: route hardcoded Blacksmith labels through the runner variables

[teamleaderleo]

Several jobs hardcoded a Blacksmith label with no variable. During the 2026-09-29 Blacksmith stall, pointing LINUX_RUNNER / MACOS_RUNNER_* at GitHub-hosted runners or the owned minis couldn't move them, and reruns stayed queued on the dead pool. Affected: iOS E2E's route job and 10 other Linux jobs across cmux-tui, cmux-cloud-cli and ios-e2e, plus two iOS E2E macOS lanes.

These jobs now read the same variables as every other job. Each one checks the standard fork-PR branch first, so fork code never follows a variable onto a self-hosted runner (test_ci_fork_runner_routing). When the variables are unset, behaviour is unchanged. This PR is needed before a single overflow switch can move every job.

Testing: scripts/verify-local.py --all passes 15/15. These pass: test_ci_fork_runner_routing, test_ci_linux_guard_routing, test_runner_label_policy, test_ci_workflow_guards_are_wired.

🤖 Generated with Claude Code

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Summary by cubic

Routes the hardcoded Blacksmith runner labels through LINUX_RUNNER and MACOS_RUNNER_* so those jobs can switch pools during a Blacksmith outage.

Bug Fixes

• 13 jobs across cmux-tui, cmux-cloud-cli, and ios-e2e hardcoded a Blacksmith label, so pointing the runner variables at alternatives couldn't move them when the pool stalled on 2026-09-29.
• Reruns on those jobs stayed queued on the dead pool instead of following the variables.
• Each job now checks the fork-PR branch first, so fork code never follows a variable onto a self-hosted runner.
• When the variables are unset, behavior is unchanged.

^{Written for commit 28e2875. Summary will update on new commits.}

Summary by CodeRabbit

• Chores
  • Updated automated validation workflows to select runners based on repository and pull request context, using configured runners when available and fallback runners otherwise. Non-owned repositories continue to use GitHub-hosted runners. These updates apply across command-line, terminal interface, and iOS end-to-end checks.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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📥 Commits

Reviewing files that changed from the base of the PR and between f5b715c and 28e2875.

📒 Files selected for processing (3)

• .github/workflows/cmux-cloud-cli.yml
• .github/workflows/cmux-tui.yml
• .github/workflows/ios-e2e.yml

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[github-actions]

Merge receipt for 28e287505d, merged 2026-09-29 12:32:43 UTC

• Not verified at merge: seven-language live conformance (in progress)
• Verified: ci-status, Web complexity, web-validation, CI fast guards, CI timing, Fast static checks, GhosttyKit release check, guards (18), ios-e2e-status, protocol contract, route, test, and 2 more
• Skipped by policy: ${{ matrix.language }} consumer, ${{ matrix.language }} package, backend, browser, Claude wrapper regressions, Dogfood build #​${{ github.event.pull_request.number }}, ios-e2e, linux-preflight, mac-host, macos, macOS admission gate, remote-daemon, and 7 more
• Full suite: runs on main after merge.