fix(#552): add DCO sign-off prohibition to fix and code agent definitions

[fullsend-ai-coder]

Summary

• Add explicit git commit -s / Signed-off-by prohibition to the Constraints section of both agents/fix.md and agents/code.md
• The constraint was previously only in the skills (code-implementation/SKILL.md and fix-review/SKILL.md), which appear later in context and were sometimes overridden by the target repo's earlier positive DCO mention
• Placing the rule in the agent definition ensures it appears early in context where the model is most likely to internalize it

Context

Multiple fix agent runs across fullsend-ai/fullsend were rejected by the post-script for including Signed-off-by trailers (PRs #5732, #6568, #6616, #6744), wasting agent runs. The skill-level prohibition remains as a second layer of defense.

Testing

• Verified constraint text appears in both agents/code.md and agents/fix.md via grep
• Ran hack/lint-agent-docs — all checks pass
• Secret scan clean
• No production code or test changes needed — this is a docs-only change to agent definitions

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Closes #552

Post-script verification

• Branch is not main/master (agent/552-prohibit-dco-trailers)
• Secret scan passed (gitleaks — 802bbd4fcade695aa6ee12fb07ba26fdb2008979..HEAD)
• PR body secret scan passed (gitleaks — no-git)

[fullsend-ai-review]

🤖 Finished Review · ✅ Success · Started 2:12 PM UTC · Completed 2:25 PM UTC

Commit: fef7b7d · View workflow run →

Runtime: claude · Model: opus → claude-opus-4-6 · Effort: high · Cost: $2.35

[fullsend-ai-review]

Risk Assessment: moderate (2/5)

Details

Docs-only change with minimal surface area (6 lines, 2 files, small blast radius) to protected agent definition files; moderate risk due to multi-author contention and elevated fix/revert history in target files, but straightforward scope and complete acceptance criteria coverage reduce overall risk.

[fullsend-ai-review]

Review

Findings

Medium

• [protected-path] agents/code.md, agents/fix.md — Both modified files are under the agents/ protected path. The PR links to issue Fix and code agent definitions should explicitly prohibit git commit -s and Signed-off-by trailers #552 and provides clear rationale for the change (preventing wasted agent runs from repeated DCO trailer rejections across PRs #5732, #6568, #6616, #6744). Human approval is always required for protected-path changes, regardless of context.