docs: add a GitHub Copilot app walkthrough #2431

Problem

The docs don't explain how to use Squad in the GitHub Copilot app.

• Choose your interface lists the Copilot CLI, VS Code, the Squad CLI, the SDK and @copilot, but not the app.
• Using Squad across clients has walkthroughs for six other clients, but not the app.
• How Squad behaves in the app is described only in the coordinator's spawn reference (.squad-templates/spawn-reference.md, "Sub-Sessions (Copilot App Mode)") and in the changelog entry for feat(coordinator): spawn cast members as sub-sessions in Copilot App #1385. Integration with the github copilot app #1118 asked how Squad works with the app and was answered only in that thread.

What a new user ran into (2026-11-08)

1. "Custom agent 'squad' not found." A new worktree session failed with session construction failed: Custom agent 'squad' not found. The files from squad init hadn't been committed and pushed, so the worktree had no .github/agents/squad.agent.md. Pushing first, or starting the session on the existing branch, fixed it.
2. Casting stopped after the roster was approved. The setup agent stopped without changing any files and showed "Squad coordinator (squad.agent.md) appears to be missing or truncated. The canary token was not found." That is the exact wording of the canary check Squad shipped before v0.13.1 ([New Session] squad agent md did not fully read #1461, fixed by fix: three-state coordinator canary + HEAD canary (Closes #1461) #1517). The old check stops every agent the coordinator spawns, because spawned agents never carry the coordinator canary. A stale copy of an old copilot-instructions.md was probably in the repository or in the user's personal Copilot instructions.
3. Agents showed up in two places. Some appeared as sub-sessions in the sidebar and others as background agents, with nothing explaining why. This is by design: the coordinator uses create_session for work that makes commits and task for read-only work.

Deliverable

A walkthrough page for the GitHub Copilot app that covers:

1. Installing the Squad CLI and running squad init, linking to Installation.
2. Committing and pushing the squad init output before starting worktree sessions, or starting the session on the existing branch instead. Check whether the app builds worktrees from the local or the remote branch and word the step to match.
3. Opening the repository, starting a session, choosing the Squad agent (/agent squad), and describing the project so the coordinator can cast the team.
4. Where agents show up: sub-sessions (nested in the sidebar, each with its own worktree) for work that makes commits, and background agents for read-only work. Include the limits: one level of nesting and about 4–5 sub-sessions at a time.
5. What the app's model, effort and context-window picker controls, and how it interacts with Squad's per-agent model selection (see Github Copilot Desktop App chat model overrides Squad defaultModel for spawned agents #2258).
6. A troubleshooting section covering the three problems above.
7. Adding the app to Choose your interface (the list and the feature matrix), the host feature matrix and Using Squad across clients.

Acceptance criteria

• Every step is checked in the current GitHub Copilot app, not inferred from the coordinator templates.
• The docs build and the docs checks pass.
• The install step works on the day the page ships. Today the Homebrew cask and the WinGet manifest for 1.1.0 download from bradygaster/squad/releases, which has returned 404 since the repository move. The same assets exist under github/squad/releases.

Out of scope

A page presenting Squad as a "software factory" (gh-aw workflows, squad watch --execute, Ralph, @copilot). That needs a maintainer decision first: #2430.