ci: publish to pkg.pr.new on push to main

[Brooooooklyn]

Summary

• Trigger the existing pkg.pr.new workflow on every push to main, in addition to the pkg.pr.new PR label and workflow_dispatch.
• Keep the one-shot PR-label behavior (no rebuild on every PR commit); main always rebuilds.
• Update concurrency comments so a newer main push cancels an in-flight main publish.

Test plan

• After merge, push (or wait for the merge commit) on main and confirm the Publish to pkg.pr.new workflow runs.
• Confirm the publish job succeeds and the package is available at the pkg.pr.new main URL.
• Confirm labeled-PR preview still works when the pkg.pr.new label is added.

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Note

Low Risk
Workflow-only change; increases CI load on main but does not touch application or release auth logic.

Overview
Extends the Publish to pkg.pr.new workflow so every push to main runs the same 8-platform build and pkg.pr.new publish as labeled PR previews and manual dispatch, giving a continuous main canary install URL.

Build and Publish preview job if conditions now include github.event_name == 'push'; PR behavior is unchanged (still only when the pkg.pr.new label is added, not on every PR commit). Comments document main always rebuilding and clarify concurrency so a newer main push cancels an in-flight main publish. reusable-build.yml comment only notes main canary usage.

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