ci: default the seed adoption kill switch to on

[teamleaderleo]

Summary

No pull request has adopted the DerivedData seed since #14081. The Adopt the nightly DerivedData seed step in compile admission was skipped on every run. That includes #14081's own three PR runs on Blacksmith 15, Blacksmith 26 and hosted 26, and PR runs from 2026-09-24 such as jobs 107499745573 and 107500740029. So every PR compile has been a full cold build.

The cause is the kill switch vars.CI_ADMISSION_SEED_DERIVED_DATA != '0'. An unset repository variable evaluates to null, not '', and Actions compares null with '0' as the numbers 0 and 0. That makes the term false while the variable is unset, which inverts the intended "unset means on".

A throwaway probe workflow, run on exp/seed-if-probe (job 107503109056), printed:

V_SEED: null
V_T_SEED: false      # vars.CI_ADMISSION_SEED_DERIVED_DATA != '0'
V_T_BACKEND: true    # the cache-backend term

It also showed that a step gated on that term alone is skipped, while one gated on the backend term runs.

The step now uses (vars.CI_ADMISSION_SEED_DERIVED_DATA || '1') != '0'. That is on while the variable is unset and off only when it is 0. This is the only variable compared with '0' in .github/workflows.

Adoption still needs a seed to exist. #14146 fixes the seeder's save step, and #14142 fixes the compile failure that broke the seed builds on Blacksmith macOS 15.

Testing

• The first commit only changes tests, and both fail on main:
  • test_adoption_is_optional_and_limited_to_pull_requests asserted the buggy expression and now requires the default.
  • The new test_no_workflow_compares_a_bare_variable_with_zero rejects any vars.X ==/!= '0' in a workflow.
• The second commit adds the fix. tests/test_seed_derived_data.py passes (10 tests).
• actionlint is clean, tests/test_ci_self_hosted_guard.sh passes, and every Python test that reads ci-macos.yml passes, except test_check_ghostty_zig_workflows.py, which can't import bashlex in this environment.

Checklist

• I added or updated tests for behavior changes
• I updated docs/changelog if needed

🤖 Generated with Claude Code

https://claude.ai/code/session_01EduXdN9PKnGsMQztJK7WeE

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Summary by cubic

Defaults the seed adoption kill switch to on so pull request compiles no longer skip the nightly DerivedData seed.

An unset repository variable is null, and Actions compares null with '0' as the numbers 0 and 0, so the old vars.CI_ADMISSION_SEED_DERIVED_DATA != '0' was false while unset — every PR compile was a full cold build. The step now uses (vars.CI_ADMISSION_SEED_DERIVED_DATA || '1') != '0', which is on while unset and off only when the variable is 0.

• Updates the adoption test to require the fixed expression and adds a test that rejects any workflow comparing a bare vars.X with '0'.
• Seed adoption still needs a seed to exist; the seeder save fix and the compile failure fix are separate PRs.

^{Written for commit f026e8c. Summary will update on new commits.}

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• .github/workflows/ci-macos.yml
• tests/test_seed_derived_data.py

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