Fix Canvas Extensions install URLs for Copilot app

[jamesmontemagno]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• I have read and followed the Guidance for submissions involving paid services.
• My contribution adds a new instruction, prompt, agent, skill, or workflow file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, agent, skill, or workflow with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.
• I am targeting the staged branch for this pull request.

---

Description

Canvas extension install links on the website were copied as SHA-pinned tree/<sha>/... URLs, which do not work reliably in extension install flows for nested extension paths. This updates the generated and rendered install links to use stable tree/main/... URLs so copied links are installable in the GitHub Copilot app flow.

The Extensions page now also includes explicit app-native install guidance and a concrete example prompt. The action label and toast text were updated to make it clear that the button copies an install URL.

---

Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New plugin.
• New skill file.
• New agentic workflow.
• Update to existing instruction, prompt, agent, plugin, skill, or workflow.
• Other (please specify):

---

Additional Notes

• This change is scoped to the Canvas Extensions website experience.
• No source issue reference was provided for automatic closing.

---

By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.

[github-actions]

⚠️ External plugin PR checks require submitter fixes

• Changed entries detected: 4
• Workflow state label: requires-submitter-fixes

Per-plugin quality summary

Plugin	skill-validator	install smoke test	overall	source tree
modernize-dotnet	not_run	not_run	infra_error	repository
sonarqube	fail	pass	fail	2.0.0
ui5	pass	pass	pass	9b3d7d80356f687725f9584988e4038dbead0d53
ui5-typescript-conversion	pass	pass	pass	9b3d7d80356f687725f9584988e4038dbead0d53

• modernize-dotnet: skill-validator=not_run, install-smoke=not_run, overall=infra_error
• sonarqube: skill-validator=fail, install-smoke=pass, overall=fail
• ui5: skill-validator=pass, install-smoke=pass, overall=pass
• ui5-typescript-conversion: skill-validator=pass, install-smoke=pass, overall=pass

[Copilot]

Copilot wasn't able to review this pull request because it exceeds the maximum number of lines (20,000). Try reducing the number of changed lines and requesting a review from Copilot again.

[aaronpowell]

@copilot resolve the merge conflicts in this pull request