Add canvas previews, external extension links, and release notes showcase

[jamesmontemagno]

Summary

This PR expands the canvas extension experience end-to-end across the repo website, extension metadata generation, and extension runtime behavior.

Website and data pipeline updates

• Added extension preview image support in website data generation and rendering.
• Added assets/preview.png convention support with safe URL encoding for asset paths.
• Added responsive extension thumbnails and click-to-enlarge preview modal.
• Added support for external extension entries (including per-entry installUrl, sourceUrl, and image support).
• Added extensions/external.json registry and wired external entries into website extensions listings.
• Added external preview asset support and included Coffilot example entry.
• Added homepage link to https://github.com/github/awesome-copilot/.

New/updated canvas extensions

• Added backlog-swipe-triage extension and hardened behavior based on review feedback:
  • pinned dependency versions
  • better async error handling
  • HTML escaping improvements
  • serialized state writes
  • request size limits + 413 handling
  • clearer session state semantics
• Added release-notes-showcase canvas extension.

Release notes showcase improvements

• Made the canvas repo-aware (resolves active repository/session context correctly).
• Added release source controls for:
  • loading a selected tag
  • drafting unreleased notes from latest tag to HEAD
• Moved Release source controls to top-level placement above Top hits.
• Improved unreleased draft generation to incorporate:
  • commits since latest tag
  • merged pull requests since latest tag
  • closed issues since latest tag
• Removed synthetic/fake defaults:
  • default canvas state is now explicit no-data until real release data is loaded
  • no fabricated sections/contributors/highlights in baseline state

Runtime/install ergonomics

• Addressed duplicate extension-provider conflict for backlog-swipe-triage (user: vs project:).
• Verified user-scope extension installation flow and reload behavior for cross-repo usage.

Branch/target

• Base branch is staged.
• PR includes only intended canvas/website/extension changes related to previews, external listings, and release-notes canvas work.

[github-actions]

⚠️ This PR targets main, but PRs should target staged.

The main branch is auto-published from staged and should not receive direct PRs.
Please close this PR and re-open it against the staged branch.

You can change the base branch using the Edit button at the top of this PR,
or run: gh pr edit 1987 --base staged

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds extension preview imagery support to the website, and introduces a new “Backlog Swipe Triage” canvas extension.

Changes:

• Render extension cards with optional thumbnail images + placeholder styling.
• Generate extension metadata that includes imageUrl/assetPath derived from extensions/*/assets.
• Add new backlog-swipe-triage extension (server-backed canvas) with docs.

Show a summary per file

File	Description
website/src/styles/global.css	Updates resource card layout to support a thumbnail + placeholder styling.
website/src/scripts/pages/extensions-render.ts	Renders extension thumbnails (or placeholder) in the extensions list HTML.
extensions/backlog-swipe-triage/package.json	Adds a new extension package using the Copilot SDK.
extensions/backlog-swipe-triage/extension.mjs	Implements the swipe-triage canvas with local HTTP server, storage, and GitHub issue actions.
extensions/backlog-swipe-triage/README.md	Documents expected assets for extension preview imagery.
eng/generate-website-data.mjs	Detects extension screenshots under assets/ and emits imageUrl/assetPath in generated metadata.

Copilot's findings

Files not reviewed (1)

• extensions/backlog-swipe-triage/package-lock.json: Generated file

• Files reviewed: 6/9 changed files
• Comments generated: 9

[jamesmontemagno]

[github-actions]

✅ Base branch is now set correctly.

Removing the prior block because this PR no longer targets main.

[github-actions]

⚠️ This PR targets main, but PRs should target staged.

The main branch is auto-published from staged and should not receive direct PRs.
Please close this PR and re-open it against the staged branch.

You can change the base branch using the Edit button at the top of this PR,
or run: gh pr edit 1987 --base staged

[github-actions]

✅ Base branch is now set correctly.

Removing the prior block because this PR no longer targets main.