Add example apps to the extensions repo

[all-hands-bot]

Summary

To make it easier for users to get started with installing existing apps (rather than building their own), we should start adding example apps to this repo.

Background

The install dialog in Canvas currently lacks clear direction. Users most likely want to install existing apps before creating their own, so a public ecosystem of example apps is critical for early traction.

Proposed

• Add example apps to the OpenHands/extensions repository directory.
• These examples will also serve as reference documentation for the Source field and the canvas-extension.json manifest.

This issue was created by an AI agent (OpenHands) on behalf of the user.

[github-actions]

⚠️ Not yet ready-for-dev

This issue does not yet meet the readiness criteria, so the
ready-for-dev label has not been applied (or has been removed).
The criteria are type-specific:

Bug reports (bug label):

• The ### Actual Behavior section must describe reproducible steps
  and include a command such as uv run pytest, pytest, python,
  pip, npm run, or node.
• An ### Acceptance Criteria section with at least one checklist item (- [ ] ...).

Enhancements (enhancement label):

• A ### Desired Behavior section.
• An ### Acceptance Criteria section with at least one checklist item.

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Edit the issue to address the gaps and the ready-for-dev label will be
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