[docs] Update documentation for safe-outputs concurrency-group

[github-actions]

Documentation Updates - 2026-03-01

This PR documents the concurrency-group field for safe-outputs: frontmatter, merged in #18993.

Features Documented

• safe-outputs.concurrency-group — new field to control concurrency on the compiled safe_outputs job (from Add concurrency-group support to safe_outputs job #18993)

Changes Made

• Updated docs/src/content/docs/reference/safe-outputs.md:
  • Added new Safe Outputs Job Concurrency (concurrency-group:) section under Global Configuration Options documenting the field, its queuing semantics (cancel-in-progress: false), and an example with a GitHub Actions expression
• Updated docs/src/content/docs/reference/concurrency.md:
  • Added Safe Outputs Job Concurrency section explaining the independent safe_outputs job, when to use concurrency-group, and a link to the safe-outputs reference
  • Added Safe Outputs to the Related Documentation list

Merged PRs Referenced

• Add concurrency-group support to safe_outputs job #18993 — Add concurrency-group support to safe_outputs job

Not Documented (no user-facing docs needed)

• Fix git push failures misattributed as "Failed to apply patch" #18987 — Fix git push errors misattributed as "Failed to apply patch" (internal bug fix)
• Add agentic-workflows label to fallback issues in create_pull_request.cjs #18986 — Add agentic-workflows label to fallback issues (internal behavior)
• Remove automatic contents: read from agent job permissions #18976 — Remove automatic contents: read from agent job (undocumented implementation detail; change aligns with least-privilege model already described in permissions.md)
• fix: explicit sparse-checkout cone mode for .github/.agents to fix runtime-import on nested paths #18973 — Fix sparse-checkout cone mode for nested paths (internal fix)
• [docs] Update dictation skill instructions #18999 — Update dictation skill instructions (docs-only, no reference docs needed)

Open Documentation Issues

• Provider-based authentication for Claude, Copilot, Codex. #16498 (Provider-based auth for Claude, Copilot, Codex) — complex design decision; not yet implemented, not addressed in this run
• [Daily Report] Daily Status Report — 2026-02-22 #17676 (Daily Status Report) — auto-generated status issue, not a documentation gap

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