Feature request: canvas/extension hook to augment built-in issue viewer with sub-issues

## Summary
The current extension surface supports extension-owned canvases and tools, but does not expose a documented way to augment the built-in Copilot issue viewer with custom sections (for example, GitHub sub-issue hierarchy).

## Request
Please consider adding an official extension/SDK hook for issue-viewer augmentation so extensions can contribute read-only (or action-capable) panels within issue sessions.

## Use case
Teams using GitHub sub-issues need parent + ordered child context in issue sessions. Today, the workaround is to build a separate custom canvas, which duplicates context and requires extra UX work.

## Suggested shape (example)
- Hook/event to provide issue-view model enrichment for current issue context.
- Extension-contributed view blocks in built-in issue viewer (scoped + permission-aware).
- Optional actions to open related issues (GitHub / Copilot issue session).

## Why SDK-level request
Even if product-native support lands, an extension hook enables repository- or organization-specific workflow customization without depending on undocumented internals.

## Related product feedback
https://github.com/orgs/community/discussions/199436

Start by reviewing the existing extension surface for extension-owned canvases and tools, then compare it with the built-in Copilot issue viewer described here. Done would require a documented, scoped hook for issue-viewer augmentation, with permission-aware panels and optional related-issue actions. The issue names no files or tests.

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