Phase 1: fail closed across verifier evidence paths

[pengfei-threemoonslab]

Summary

This PR is the Phase 1 trustworthy-candidate hardening pass. It makes non-mergeable verifier states operationally fail closed and closes several evidence paths that could previously degrade silently.

Changes

• Fail both Agents Shipgate workflows on every non-mergeable verifier verdict:
  • blocked
  • human_review_required
  • insufficient_evidence
  • unknown
• Emit source warnings for parsed-but-invalid .mcp.json files instead of silently returning an empty tool surface.
• Treat unrecognized n8n MCP Client selection modes as an unproven wildcard surface and route them to review.
• Convert enabled plugin validation/runtime failures into source-warning evidence while preserving advisory CLI exit behavior.
• Fail closed when the verifier cannot collect working-tree diff context.
• Route semantic diff-reference degradation through the typed evidence action rather than matching CLI command/path strings.
• Correct the handoff v5 documentation and control.next_action guidance.
• Update workflow contract tests for the new four-verdict policy and make the help-output assertion robust to Rich line wrapping.

Validation

• Ruff: passed on all tracked Python files.
• Generated-schema consistency: passed.
• Static-adapter suite: 400 passed.
• Remaining non-performance suite: 3,468 passed, 5 skipped.
• Committed-tree verification receipt:
  sha256:4e03af1348814cdadf9393dd42c41f3606b8209807be578358069628045e1b78

Trust and release state

The committed-tree verifier completed successfully, but the candidate is intentionally not mergeable without human review:

• control.state = human_review_required
• control.must_stop = true
• gate.merge_verdict = insufficient_evidence
• Both workflow files are correctly recognized as touched release trust roots.
• shipgate.yaml#agent_bindings.root still requires a human-reviewed exact source_id and object declaration.

This PR does not suppress findings, lower severity, expand a baseline/waiver, weaken CI, or invent binding/authority evidence.

Required human action before merge

1. Review the two protected workflow changes as release trust-root changes.
2. Determine and author the real root binding values in shipgate.yaml#agent_bindings.root; do not infer or auto-fill them.
3. Rerun:

agents-shipgate verify --base origin/main --head HEAD --json

The resulting verifier artifact remains the authority for merge readiness.