docs: close non-Copilot engine parity gaps for gh aw init and custom engine frontmatter

[Copilot]

cli.md told non-Copilot users that gh aw init "skips these Copilot-specific artifacts" without saying what they get instead, and the reported flat engine: custom form does not exist anywhere in the docs or examples. Documentation-only fix.

setup/cli.md — new "Initializing for non-Copilot engines" subsection

• Per-artifact parity table: .gitattributes and the dispatcher skill (.github/skills/agentic-workflows/SKILL.md) are created for every engine; only .github/agents/agentic-workflows.md and the MCP wiring (.github/mcp.json + copilot-setup-steps.yml) are Copilot-gated.
• Replacement path for each skipped artifact: use the dispatcher skill or author an agent file in the target agent's own format; register gh aw mcp-server manually via GH-AW as an MCP Server.
• States the engine-independent next steps (frontmatter engine: + that engine's auth secret) and that the init-time engine does not constrain per-workflow engine choice.
• Existing gating sentence now links to the new subsection.

reference/engines.md — custom engine form

Note clarifying that string-form engine: accepts only built-in IDs; third-party engines always use nested engine.id + an imported definition. Verified against the compiler:

engine: custom   # error: invalid engine: custom. Valid engines are: claude, codex, copilot, gemini, pi.

engine:
  id: example-engine
imports:
  - owner/repository/.github/workflows/example-engine.md@v1.2.3

examples/index.md — engine portability

Explains the Copilot-skewed example distribution and that examples port across engines by changing engine: and its auth secret, flagging engine.agent / engine.harness as non-portable with a pointer to the feature comparison table.

• Fixes [deep-report] Document non-Copilot init/example parity gaps (Claude/Codex/custom engine) #53791

[github-actions]

PR Triage

• Category: docs · Risk: low
• Score: 30/100 (impact 10, urgency 5, quality 15)
• Recommended action: defer

Small doc parity fix (23 lines), draft, no urgency. Merge when convenient.

Generated by 🔧 PR Triage Agent · auto · 58.5 AIC · ⌖ 2.15 AIC · ⊞ 8.3K · ◷

[Copilot]

Pull request overview

Clarifies engine portability and non-Copilot initialization behavior across the documentation.

Changes:

• Documents artifacts created or skipped for non-Copilot engines.
• Clarifies custom engine configuration.
• Adds guidance for adapting examples between engines.

Show a summary per file

File	Description
docs/src/content/docs/setup/cli.md	Documents non-Copilot initialization behavior.
docs/src/content/docs/reference/engines.md	Clarifies custom engine selection.
docs/src/content/docs/examples/index.md	Adds engine portability guidance.

Review details

Suppressed comments (1)

docs/src/content/docs/setup/cli.md:180

• Changing only engine: and authentication is not sufficient for every example. The engine reference explicitly notes that switching may also require different tools, model names, or network access, beyond the engine.agent and engine.harness cases. The categorical adaptation claim should direct users to review all engine-specific settings.

The engine chosen at `init` time does not restrict workflows: every workflow selects its own engine in frontmatter, and example workflows written for one engine can be adapted to another by changing `engine:` and its authentication secret.

• Files reviewed: 3/3 changed files
• Comments generated: 3
• Review effort level: Balanced

[github-actions]

🎉 This pull request is included in a new release.

Release: v0.87.2