docs: fix stale engine template docs, add TODO for test engine hardcode

[jamesadevine]

Summary

Follow-up to #286 — fixes two items missed in the merge:

Changes

1. AGENTS.md: Replace stale {{ copilot_ado_env }} section with correct {{ engine_env }} docs. The template marker was renamed but the docs still described the old semantics (ADO token passthrough). The actual {{ engine_env }} generates engine-specific env vars (GITHUB_TOKEN, OTEL config) via Engine::env().

2. CompileContext::for_test(): Add TODO comment flagging that Engine::Copilot is hardcoded. Currently correct (only engine), but will need to resolve from front_matter.engine when additional engines are added.

Testing

All 930 tests pass.

[github-actions]

🔍 Rust PR Review

Summary: Looks good — clean docs fix with accurate content, and a well-placed TODO.

Findings

✅ What Looks Good

• AGENTS.md update is accurate: The new {{ engine_env }} docs correctly reflect what Engine::Copilot::env() actually generates (GITHUB_TOKEN, GITHUB_READ_ONLY, COPILOT_OTEL_*). The code in src/engine.rs matches the docs exactly, and {{ acquire_ado_token }} correctly handling ADO tokens separately is well-explained.
• No stale references: copilot_ado_env is fully removed — no dangling references in templates or code.
• TODO placement: The comment in for_test() is accurate and actionable — the current hardcode of Engine::Copilot is safe since it's the only variant.

⚠️ Suggestion

• [src/compile/extensions/mod.rs:174] for_test_with_org() has the same Engine::Copilot hardcode but the TODO only covers for_test(). Minor, but worth adding the same note there for consistency when this comes up for real.

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