feat: add triage evidence card

[GodsBoy]

Summary

• add compact triage evidence cards for issue comments
• show duplicate candidate clusters while keeping issues open by default
• reuse the same evidence card style in playground issue previews
• add strategy, requirements, and plan docs for the trustworthy triage track

Test plan

• pnpm build
• pnpm lint
• .\node_modules.bin\vitest.CMD run
• git diff --check
• git diff --cached --check

Post-Deploy Monitoring & Validation

Validation window: first 24 hours after deployment. Owner: repository maintainer.

Expected healthy signals:

• New issue triage comments include a compact evidence card.
• Possible duplicate issues stay open and receive candidate links.
• Playground issue previews return evidence-card output.
• Webhook handler logs continue to show successful issue triage events.

Failure signals:

• Maintainers report duplicate comments as too noisy or misleading.
• Issue triage comments fail to post after classification.
• Playground issue previews return provider errors for otherwise valid inputs.

Monitoring checks:

• Search application logs for Triaging issue #, classified as, flagged as possible duplicate, and Playground AI error.
• Watch GitHub webhook delivery responses for 5xx errors on issues.opened.
• Review the first few live issue comments for compact card formatting and candidate wording.

Rollback trigger:

• If issue comments become misleading or webhook failures increase, revert this PR and redeploy the previous main commit.