[docs] Update engines.md with Copilot BYOK provider env vars

[github-actions]

Documentation Updates — April 19, 2026

This PR updates reference/engines.md with the COPILOT_PROVIDER_* environment variable documentation that has been missing since Copilot CLI 1.0.32 shipped BYOK support.

Features Documented

• Copilot BYOK provider configuration (COPILOT_PROVIDER_* env vars) — all 8 variables introduced in Copilot CLI 1.0.32 that configure an external model provider when using features.byok-copilot: true (from open issue Provider-based authentication for Claude, Copilot, Codex. #16498)

📝 Detailed Changes & References

Changes Made

• Added new #### Copilot BYOK Provider Configuration subsection to docs/src/content/docs/reference/engines.md under the existing "Engine Environment Variables → Custom API Endpoints via Environment Variables" section
• Documents all 8 COPILOT_PROVIDER_* variables in a reference table
• Includes a complete Anthropic provider example showing features.byok-copilot: true, engine.env, and network.allowed working together
• Links to the GitHub Copilot external docs for enterprise policy details

Merged PRs Referenced

• docs(constants): sync README with newly exported guard-policy, BYOK, and AWF version constants #27111 — docs(constants): sync README with newly exported guard-policy, BYOK, and AWF version constants (confirmed BYOK constants exist)
• [docs] Update dictation skill glossary with missing terms #27153 — [docs] Update dictation skill glossary with missing terms (today's merged docs PR — no engine changes)

Why auth.mdx was not updated

docs/src/content/docs/reference/auth.mdx has disable-agentic-editing: true in its frontmatter, indicating this file should not be modified by agents.

Skipped Issues

Issue	Reason
#20391 — Stale v0.45.5 blog post links (115 occurrences)	Requires bulk link-update automation across ~18 blog posts; out of scope for a single content PR
#25383 — Playwright smoke test for editor playground	Testing infrastructure gap, not a content documentation gap
#27143 Copilot CLI 1.0.32 hooks/--add-github-mcp-tool*	Cannot verify Copilot CLI-level flags from this codebase; requires a separate verified PR

Notes

• The COPILOT_PROVIDER_* variable list is sourced from the Daily Documentation Quality Report (issue [Daily Report] Documentation Quality Report — April 19, 2026 #27164) which confirmed these variables against Copilot CLI 1.0.32 (issue [ca] CLI Version Updates: Claude Code 2.1.114, Copilot CLI 1.0.32 #27143). The variables are not present in this repo's source (they are consumed by the Copilot CLI binary directly), which is why they required a doc-only addition.
• Issue Provider-based authentication for Claude, Copilot, Codex. #16498 remains partially open — the Claude/Codex provider-based auth aspect is not yet addressed; only the Copilot BYOK env vars have been documented.

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• expires on Apr 20, 2026, 10:57 AM UTC

[github-actions]

👋 Great to see the Daily Documentation Updater keeping things current — adding the COPILOT_PROVIDER_* env var reference table and BYOK example to engines.md fills a real gap for users on Copilot CLI 1.0.32+. The PR body is thorough and well-structured.

One thing to flag: even for docs-only changes, the contributing guidelines call for make agent-finish to be run before completing work. There's no indication in this PR that it was run.

If make agent-finish isn't appropriate for a fully automated docs PR, at minimum make fmt and make recompile should be run to confirm formatting and compiled workflow consistency.

Run the following validation steps for this documentation PR:
1. Run `make fmt` to ensure all files are properly formatted.
2. Run `make recompile` to verify compiled workflow files are consistent with source markdown.
3. If any files are modified by the above commands, commit and push the changes.
4. Confirm that `make agent-finish` passes without errors.

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