docs(#530): add backlink guidance for new problem docs

[fullsend-ai-coder]

When a new problem doc covers a topic already discussed in existing docs, the author should search docs/problems/ for references to the concept and add reciprocal backlinks in the same PR. This codifies a pattern the maintainer had to apply manually on PR fullsend-ai#3064 and sets the expectation for both authors and reviewers.

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Closes #530

Post-script verification

• Branch is not main/master (agent/530-backlink-guidance)
• Secret scan passed (gitleaks — b776cf6006a12016eaa21412dc65a3001100e801..HEAD)
• Pre-commit hooks passed (authoritative run on runner)
• Tests ran inside sandbox

[fullsend-ai-review]

🤖 Review · ❌ Terminated · Started 5:36 PM UTC · Ended 5:40 PM UTC
Commit: b776cf6 · View workflow run →

[fullsend-ai-review]

Review — approve

Reviewed: 3e8abce (1 file changed, +1 −0)

Summary

Single-line addition to AGENTS.md that codifies backlink guidance for new problem docs, matching the wording proposed in #530 verbatim. Placed correctly in the "How to work in this repo" bullet list, directly after the existing "link it from README.md" instruction.

Dimensions evaluated

Dimension	Result
Correctness	✅ Guidance is technically accurate and internally consistent with existing AGENTS.md rules. No contradictions.
Security	✅ Clean ASCII, no injection patterns, no secrets, no permission changes.
Intent & coherence	✅ Traces to issue #530. Scope matches exactly — no creep.
Style & conventions	✅ Bullet formatting, backtick usage, punctuation, and sentence structure match surrounding lines.
Docs currency	⏭ Not applicable (change is documentation itself).
Cross-repo contracts	⏭ Not applicable (no API/interface changes).

No findings.

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Protected paths detected — this PR modifies files under one or more
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Protected files in this PR:

• AGENTS.md

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Labels: PR adds contributor guidance to AGENTS.md