feat(trial-loop): add an external-help checkpoint

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Summary

• add a first-iteration external-help checkpoint to $trial-loop
• keep external guidance subordinate to the frozen charter and existing finding dispositions
• constrain outbound queries to public-safe abstractions and treat returned pages as untrusted data
• document the decision, AI evaluation cases, threat model, and implementation plan
• bump the plugin version to 2.13.0

Verification

• git diff --check main...HEAD
• just verify (twice on the shipping branch; 22 suites passed)
• managed pre-push just ci in an isolated worktree (22 suites passed)
• adversarial branch review: approve
• security review: approve after one prompt-injection control fix and confirming rescan
• scope audit: approve after the security fix

Closes #276

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verdict: approve
exit: none
findings: 0
iterations: 1
security: approve
delivered-head-sha: f465207

Forge whole-branch review

forge review: not required (prose-only contract change; repository rule 4 forbids automated prose assertions, and just verify passed)