Project-scoped extension canvases are declared but not routable by open_canvas

## Summary

Project-scoped extensions loaded from `.github/extensions/` can initialize and register a canvas with `createCanvas()` + `joinSession()`, and the canvas appears to be declared in the runtime/system prompt. However, the canvas is not added to the runtime routing table: `open_canvas` cannot open it and `list_canvas_capabilities` does not find it.

The same canvas code works when installed as a user-scoped extension under `~/.copilot/extensions/`.

## Version

- Copilot CLI: 1.0.69
- Runtime: 1.0.66-1
- OS: Windows 11 (x64)

## Repro

1. Create a project-scoped extension under `.github/extensions//extension.mjs`.
2. Register a canvas using `createCanvas()`.
3. Start the extension session with `joinSession({ canvases: [canvas] })`.
4. In Copilot CLI, ask to open that canvas or call `open_canvas` for the canvas ID.
5. Call `list_canvas_capabilities` for the same canvas ID/name.

**Note:** Extension logs confirm full initialization — `joinSession` completes, the catalog loads, and the HTTP server responds to requests. The extension process is healthy; only the runtime routing is broken.

## Expected behavior

Project-scoped canvases registered through `joinSession({ canvases: [...] })` should be added to the runtime canvas routing table and should be openable by `open_canvas`.

## Actual behavior

- The extension initializes successfully.
- The canvas appears to be declared to the model/runtime.
- `open_canvas` fails with `connection closed`.
- `list_canvas_capabilities` returns `No canvas matches`.

## Workaround

Opening the extension's local HTTP URL directly in a browser canvas works. Installing the same extension as a user-scoped extension under `~/.copilot/extensions/` also works.

## Impact

Project-scoped Canvas extensions can be discovered/declared but cannot be opened, which blocks repo-local Canvas experiences from working without a user-scoped install or manual browser workaround.

Start with the project-scoped extension entry point at .github/extensions//extension.mjs and trace how joinSession({ canvases: [canvas] }) registrations reach the runtime routing table. Compare this path with the working user-scoped installation, then verify that open_canvas can open the canvas and list_canvas_capabilities finds it.

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