ci: install Rust toolchain in jobs that build the broker on StarSling

[kjgbot]

Summary

Three jobs on promotion PR #1939 failed right after #1946 moved them to vars.CI_LINUX_RUNNER (starsling-ubuntu-24.04). The failing jobs are test (ubuntu-latest, 22.14.0), Coverage (upload) and Test build-bun.sh (ubuntu-latest). All three failed for the same reason: the Rust compiler preinstalled on the StarSling runners is too old.

• StarSling runners preinstall rustc 1.92.0 (2025-12-08). GitHub's ubuntu-24.04 image 20261004.327 ships rustc 1.99.0. I took the 1.92 version from the Rust Tests (ubuntu-latest) job log on the same StarSling pool: stable-x86_64-unknown-linux-gnu updated - rustc 1.99.0 ... (from rustc 1.92.0 (ded5c06cf 2025-12-08)).
• The broker uses AtomicU*::try_update (crates/broker/src/node_delivery_probe.rs:359, from fix(broker): preserve node tokens across rotation races #1907, already on main). rustc 1.92 rejects it:

  error[E0658]: use of unstable library feature `atomic_try_update`
     --> crates/broker/src/node_delivery_probe.rs:359:51
  error: could not compile `agent-relay-broker` (lib) due to 1 previous error
  

• Nothing else in trunk → main: fix(broker): verify PTY turn submission (#1891) #1939 causes this. On main at 5b30a69d7, which already contains fix(broker): preserve node tokens across rotation races #1907 and ran on GitHub runners, test, Coverage and Test build-bun.sh (ubuntu-latest) all passed.
• The Rust CI jobs on trunk → main: fix(broker): verify PTY turn submission (#1891) #1939 already run dtolnay/rust-toolchain@stable and passed on the same StarSling runners. Installing the toolchain takes about 8s.

Change

Add dtolnay/rust-toolchain@4360b52568e2003a75bf9bc1d59f33a8e3fc893c # stable (the pin other workflows here already use) to the test and coverage jobs in test.yml and the test-build-script job in test-build.yml. These jobs stay on StarSling. I did not touch any runs-on or trunk gate. On macOS the step just confirms that stable is current.

The Codecov upload step has not run on StarSling yet, because the job failed before reaching it. It should be checked on the next #1939 run.

Local checks

• actionlint .github/workflows/test.yml: clean
• actionlint .github/workflows/test-build.yml: 9 shellcheck findings, all in steps this PR does not touch. The origin/trunk version of the file has the same 9.
• python3 -c 'import yaml; ...' on both files: parses

Not in this PR

Flows v2 shard 14 (broker-local-only: "Absent local work must not be dead-lettered") ran on a GitHub-hosted runner, not StarSling, so this change does not affect it. It is tracked separately.

🤖 Generated with Claude Code

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Note

Low Risk
CI-only workflow changes with no application or security logic; risk is limited to job runtime and build reliability on custom runners.

Overview
Adds an explicit Rust stable toolchain setup to CI jobs that compile the broker on StarSling Linux runners (vars.CI_LINUX_RUNNER), where preinstalled rustc 1.92 fails on broker code that needs stable Atomic*::try_update.

The new Setup Rust toolchain step (dtolnay/rust-toolchain at the same pinned SHA as rust-ci.yml) runs in test and Coverage (upload) in test.yml, and Test build-bun.sh in test-build.yml, immediately after checkout. macOS matrix legs only verify stable is already present; runner labels and trunk gates are unchanged.

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

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No issues found across 2 files

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