docs(#563): add Go concurrent fan-out error handling guidance

[fullsend-ai-coder]

Add rule to AGENTS.md Go code section: when goroutines fan out independently, collect all errors into a []error slice protected by sync.Mutex and return errors.Join(errs...). Flag single-error capture (e.g., firstErr) in fan-out patterns as a medium-severity review finding.

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Closes #563

Post-script verification

• Branch is not main/master (agent/563-go-concurrent-errors-join)
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[fullsend-ai-review]

🤖 Finished Review · ✅ Success · Started 7:32 AM UTC · Completed 7:36 AM UTC
Commit: b776cf6 · View workflow run →

[fullsend-ai-review]

Review — ✅ Approve

PR: #571 — docs(#563): add Go concurrent fan-out error handling guidance
Scope: Documentation-only (AGENTS.md, +2 lines)

Summary

Clean, well-scoped addition to the Go code section of AGENTS.md. Adds guidance for concurrent error handling in fan-out goroutine patterns: collect all errors into a []error slice protected by sync.Mutex and return errors.Join(errs...), rather than capturing only the first error. The text accurately implements the proposed change from issue #563, which was derived from a real review miss on PR fullsend-ai#2671 (upstream).

Reviewed dimensions

Dimension	Status
Correctness	✅ errors.Join (Go 1.20+) with mutex-protected slice is the idiomatic pattern for collecting all concurrent errors
Scope	✅ Matches issue #563 exactly; no out-of-scope changes
Placement	✅ Reasonable position in the "Go code" section, after the testing checklist
Style	✅ Consistent with existing AGENTS.md guidance format (bold heading, clear do/don't, review severity callout)
Security	✅ No secrets, no code changes, no injection patterns
Protected paths	✅ CODEOWNERS requires @fullsend-ai/core approval (enforced by branch protection)

Findings

No findings at low severity or above.

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Protected paths detected — this PR modifies files under one or more
protected paths. The review agent cannot approve PRs that touch these paths.
A human reviewer must approve this PR.

Protected files in this PR:

• AGENTS.md

[mysticgohan1]

Closing: Run A (baseline) complete. Artifacts archived.

[guyoron1]

Run A (baseline) complete. Artifacts archived.