Document Copilot app launcher links

[basiclines]

Summary

Adds a GitHub Copilot app docs page that showcases the hosted launcher URL contract for opening the app from links.

The new page includes:

• The customer-facing hosted launcher URL shape:

  https://github.com/copilot/app/launch?open=ENCODED_APP_LINK
  

• A complete app-link reference table for available ghapp:// targets, including home, chats, My work, repository setup, issues, pull requests, agent task resume, automations, legacy workflow fallbacks, recent workspaces, app-local sessions, session creation, and plugin setup links.

• Parameter reference tables for session creation, automation drafts/modes, and plugin setup sources.

• Copy-pastable launcher examples for repository, issue, pull request, agent task resume, and automation links.

Validation

• npm run lint-content -- --paths content/copilot/how-tos/github-copilot-app/index.md content/copilot/how-tos/github-copilot-app/open-from-github.md
• npx --no-install prettier -c content/copilot/how-tos/github-copilot-app/index.md content/copilot/how-tos/github-copilot-app/open-from-github.md
• git diff --check
• pre-commit lint-content --precommit --paths

[github-actions]

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Table of review links

Note: Please update the URL for your staging server or codespace.

The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
copilot/how-tos/github-copilot-app/index.md	fpt ghec	fpt ghec
New file: copilot/how-tos/github-copilot-app/open-from-github.md	fpt ghec	fpt ghec

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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