Collapse repeated source-mode ternary in test-smokes-built.yml into one resolve-mode job

[cderv]

test-smokes-built.yml derived the build/nightly/release source mode independently at 17 sites: 12 compound job conditions, 1 compound run-name, and 4 short release-family conditions written in a divergent short form. The two idioms already diverged — they agree today only because workflow_run forces nightly and github.event.inputs.source is null on that trigger — so a future change to mode resolution needs identical edits in 17 places, and the release family wouldn't even be found by grepping for the compound expression.

Add a resolve-mode job that computes the mode once, validates it against build/nightly/release (hard-failing otherwise), and exposes it as a job output. All 16 downstream jobs now gate on needs.resolve-mode.outputs.mode instead of re-evaluating the ternary. run-name keeps its own copy since it evaluates before any job exists and has no needs context.

resolve-mode also echoes the triggering workflow_run's id and conclusion from the event payload, since the API exposes no pointer from a workflow_run run back to its trigger — useful for diagnosing a nightly run after the fact.

Followers keep their own mode check alongside needs: [resolve-mode, ...] even though skip propagation already makes it redundant, matching the existing invariant in .claude/rules/testing/built-version-ci.md.

Tradeoffs

One extra runner job, serialized ahead of everything — roughly 15-30s per run, including nightlies. A runner outage or image failure in resolve-mode now skips all 16 jobs instead of failing at expression-evaluation time: loud rather than silent, but a new single point of failure for the whole workflow. Every non-success "Build Installers" completion (the workflow_run trigger is types: [completed], not just success) now also costs one short runner job where before nothing scheduled at all.

Verification

• actionlint .github/workflows/test-smokes-built.yml clean
• Dispatched source=build with a narrow bucket — resolve-mode resolved build, only the build family scheduled, everything else skipped, run succeeded: https://github.com/quarto-dev/quarto-cli/actions/runs/35726105621
• Dispatched source=nightly with a narrow bucket — resolve-mode resolved nightly, resolve-nightly plus all three OS legs scheduled, build/release families skipped, run succeeded: https://github.com/quarto-dev/quarto-cli/actions/runs/35726404470
• The workflow_run (post-"Build Installers") path can only be exercised after merge, since workflow_run always runs the default-branch copy of the workflow

No test file, no src/ changes, no changelog entry — internal CI plumbing with no user-visible behavior change.

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