Add `shared` to BundleTrigger::Pubsub + PubsubTriggerSpec + the trigger
identity (so toggling re-diffs) + current_trigger_identity. validate_bundle_for
now allows a `shared` pubsub trigger on a group and requires the topic
pattern's ROOT segment (events.* -> events) be a declared kind='topic'
collection; a wildcard root is a runtime match. Apply-side plumbing only —
the shared publish path + dispatch boundary land next.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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