Make Shipgate verify-first for PR gates

[pengfei-threemoonslab]

Summary

• Reorients README, quickstart, agent snippets, adoption kits, and golden PR docs around the agents-shipgate verify flow.
• Updates the capability-review PR comment so the first screen leads with merge_verdict, capability deltas, Required before merge, trust-root warnings, and artifact links while preserving release_decision.decision as the source gate.
• Tightens the adoption harness to score verify, verifier.json.merge_verdict, capability_review.top_changes[], and human-review handoff behavior.

Validation

• python -m pytest tests/test_verify.py tests/test_github_action_outputs.py tests/test_action_metadata.py
• python -m pytest tests/harness/test_detectors.py tests/harness/test_exit_criteria.py
• python -m pytest tests/test_docs_links.py tests/test_documentation_checks.py
• python -m pytest
• python -m ruff check .
• git diff --check
• PYTHONPATH=src AGENTS_SHIPGATE_AGENT_MODE=1 python -m agents_shipgate verify --preview --json (merge_verdict: unknown, because this worktree has no root shipgate.yaml)

Notes

This PR intentionally changes the default PR-comment surface for capability-review; it does not add a second gate or change report.json.release_decision.decision semantics.