[docs] Update documentation for features from 2026-07-26

[github-actions]

Summary

Documentation updates for two reference pages covering features available as of 2026-07-26.

Changes

docs/src/content/docs/reference/safe-outputs.md

• Documents the GitHub Actions expression forms accepted by github-token: secrets.NAME, needs.<job>.outputs.<name>, and steps.<id>.outputs.<name>.
• Adds an example showing how to mint a short-lived token in pre-steps: and reuse it via steps.*.outputs.* in the same job.

docs/src/content/docs/reference/sandbox.md

• Adds a new section for sandbox.agent.targets.copilot BYOK request customization.
• Documents extraHeaders, extraBodyFields, and sessionId options for routing Copilot through OpenAI-compatible proxies and gateways.
• Notes that sessionId is opt-in only; automatic injection is explicitly avoided because some strict providers reject unknown session_id fields.

Impact

• No code changes. Documentation only.
• Not breaking.

Generated by PR Description Updater for #48178 · sonnet46 · 21.7 AIC · ⌖ 7.39 AIC · ⊞ 4.8K · ◷

[gh-aw-bot]

@copilot please run the pr-finisher skill and summarize the current maintainer-ready state for this PR.

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Generated by 👨‍🍳 PR Sous Chef · gpt54 · 8.17 AIC · ⌖ 8.22 AIC · ⊞ 7.1K · ◷
_{Comment /souschef to run again}