ci: cancel superseded runs on main, not just on PRs

[ScriptedAlchemy]

Summary

Every push to main used a SHA-keyed concurrency group with no cancellation in ci.yml, docs.yml, and package-preview.yml, so a burst of merges left one queued run per commit (18 queued runs after tonight's four merges). Only the tip matters, so the groups now key on github.event.pull_request.number || github.ref with cancel-in-progress: true: a PR push still cancels the superseded PR run, and a main push (or dispatch) now cancels the superseded main run. Merge-queue entries and dispatches on other refs get their own ref-keyed group as before.

• ci.yml, package-preview.yml: ref-keyed groups, always cancel.
• docs.yml: ref-keyed group, but main stays non-cancelling so the Pages deploy job is never killed mid-deployment (workflow-level cancellation would override the job's own cancel-in-progress: false). GitHub already drops superseded pending runs in a group, so the main docs queue is at most one deep.
• release.yml is unchanged: package-release never cancels a publish in flight.
• docs/preview-packages.md, website/docs/{en,zh}/guide/distribution/preview-packages.mdx, website/docs/{en,zh}/guide/start/installation.mdx: drop the "every main commit has an installable snapshot" promise — a commit overtaken before its preview published has none; the tip always does.
• AGENTS.md: notes that the post-merge run to watch is the tip's, since a later push cancels the superseded run.

Trade-off accepted (owner request, 2026-09-06): intermediate main commits may have no completed CI/preview run.

Validation

• actionlint .github/workflows/{ci,docs,package-preview}.yml — clean.
• pnpm build && pnpm docs:site:build — green (website prose changed; dead-link and en/zh parity checks pass).
• No package code touched, skip-changeset.

Deslop

Deslop: Claude Fable 5.1, 0 edits (comments rewritten to describe the new policy; no code).

Self-review

Reviewer: gpt-5.6-sol-medium (generalPurpose). Five findings, all fixed:

1. docs.yml — workflow-level cancel-in-progress: true would override the deploy job's cancel-in-progress: false and could kill a Pages deployment mid-flight. Fixed — main runs stay non-cancelling in docs.yml; only PR runs cancel.
   2–5. website/docs/{en,zh}/guide/distribution/preview-packages.mdx and website/docs/{en,zh}/guide/start/installation.mdx still promised per-commit, uncancellable main previews. Fixed — all four pages now describe ref-keyed cancellation and tell readers to pin the tip.

Expression check per event (verified by the reviewer): pull_request → PR number; push/workflow_dispatch on main → refs/heads/main; merge_group → refs/heads/gh-readonly-queue/main/pr-N-<sha> (unique per entry). No cross-event collisions.

Second pass (same reviewer): no merge risks found.

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