revert: trunk-only CI gating (repo not in merge-train core set)

[kjgbot]

Reverts the trunk-only CI gating and the merge-train AGENTS.md snippet. This repo is not in the core merge-train set, so normal per-PR CI is restored.

🤖 Generated with Claude Code

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Note

Medium Risk
CI behavior and merge policy change for every PR (more workflows run on feature branches); no application runtime code changes, but misconfiguration could weaken intended publish-only guards if schema-publish triggers were wrong.

Overview
Reverts trunk-only CI so checks run on ordinary pull requests again instead of being limited to trunk → main (or non-PR events).

Workflows: Removes the shared job if that skipped PR jobs unless github.head_ref == 'trunk' from cloud-runtime-artifact, review-swarm, review-swarm-wrapper-guard, and surface-package. In schema-publish, the same gate is dropped from validate; npm and pages now only require github.event_name != 'pull_request' instead of the old trunk + non-PR compound condition.

Repo policy: Deletes .github/workflows/trunk-guard.yml (which failed PRs into main that were not from trunk), strips the merge-train AGENTS.md snippet (mergeable label, base trunk, no CI on feature branches), and removes the corresponding prpm.lock entry for @agent-relay/merge-train-snippet.

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