fix(ci): dogfood gate passes cancelled runs through — a cancelled matrix is not a failed one

[os-zhuang]

Fixes #3668.

Why

With cancel-in-progress on, every consecutive push cancels the in-flight run — and the dogfood matrix, as the longest job in the workflow, is almost always the one still running. The gate's catch-all *) branch turned needs.dogfood.result == "cancelled" into exit 1, painting a false red on every superseded SHA (two observed on #3660 alone, logs identical: dogfood matrix aggregate result: cancelled). False reds on a regression gate train people and agents to ignore the one check that must never be ignored.

What

The issue's prescribed one-line fix: cancelled joins the pass branch —

success|skipped|cancelled) echo "Dogfood gate satisfied ($result)." ;;

— plus an inline comment recording the evidence and the rejected alternative.

Safety premise — verified experimentally before landing, per the issue's own ask

The fix is only safe if a real shard failure can never surface as an aggregate cancelled. Ran the experiment (run 30271824408, temporary workflow since removed from the branch): a fail-fast: true matrix where shard 1 exits 1 immediately and shard 2 sleeps until fail-fast cancels it mid-run — the harshest version of "failure and cancellation coexist".

Result: shard 1 concluded failure, shard 2 concluded cancelled, and the aggregate observer printed needs.matrix.result = failure. Failure dominates. An aggregate of cancelled therefore only occurs when the whole run is stopped from outside (concurrency supersession or manual cancel) with no leg having failed — passing it masks no regression.

Notes recorded in the inline comment:

• Manual cancel of a head run now turns the gate green — the issue's accepted trade-off (the superseded-SHA case, which is the overwhelming majority, is irrelevant to merging since branch protection evaluates the latest SHA).
• if: !cancelled() on the job was deliberately rejected (as the issue prescribes): a skipped gate publishes no required-check context on that SHA — the chore(ci): cut PR wall-clock ~9.5min → ~5-6min — dedupe dogfood, shard it, cache lint's build #3622 merge-deadlock again.
• grep 'needs\..*\.result' .github/workflows/*.yml confirms this is the only aggregate gate of this shape, so no sibling occurrences to fix.

🤖 Generated with Claude Code

https://claude.ai/code/session_01ECTCrcCdZpCHw5zFSgcmGt

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