Native GitHub Copilot inline completions

[anxkhn]

Description

Add native GitHub Copilot inline completions to CodeEdit: as you type, the editor shows AI ghost text suggestions and accepts them with Tab. This is the app-level integration that ties together the inline suggestion rendering (CodeEditTextView) and the inline completion coordination layer (CodeEditSourceEditor).

What it adds:

• A Copilot language-server client and service (CopilotClient, CopilotService, CopilotProtocol) that launches and talks to the official copilot-language-server over LSP, including device-flow sign-in, status, and document sync.
• A per-document CopilotInlineCompletionProvider that conforms to the editor's InlineCompletionDelegate, maps the caret to an LSP position, requests completions, strips already-typed prefixes, and adapts results into the editor's ghost-text model.
• A CopilotBinaryResolver that locates copilot-language-server (explicit path, PATH, or common install dirs) and builds a launch environment that can find node (including nvm-managed installs), since the server is a Node script.
• A Settings page (enable toggle, device-flow sign-in UI, optional language-server path) and a status bar item reflecting Copilot state.
• Wiring in CodeFileDocument / CodeFileView to attach the provider per open document.

Design notes:

• Suggestions render as ghost text via the CodeEditTextView overlay primitive and never mutate the document until accepted.
• Disabled by default; opt-in via Settings. Requires a GitHub Copilot subscription.

Alternatives Considered

• A plugin/extension instead of a built-in. CodeEdit does not yet have an extension API surface for inline completions, and the integration needs the editor's cursor/lifecycle hooks, so a built-in (opt-in, behind settings) is the practical option today.
• Driving the suggestion lifecycle in the app rather than the editor. Rejected; the request/accept/dismiss lifecycle lives in CodeEditSourceEditor (see the dependency below), and the app only supplies a concrete provider.

Additional Context

This is the top layer of a three-repo change for native inline completions:

1. CodeEditTextView: inline suggestion (ghost text) rendering primitive (Inline suggestion (ghost text) rendering primitive CodeEditTextView#124, PR Add inline suggestion (ghost text) rendering primitive CodeEditTextView#125).
2. CodeEditSourceEditor: inline completion coordination layer (Inline completion (ghost text) coordination layer CodeEditSourceEditor#374, PR Add inline completion (ghost text) coordination layer CodeEditSourceEditor#375).
3. CodeEdit (this issue): the GitHub Copilot provider and UI.

I have a working, documented implementation. Locally it builds and runs against the two dependency branches via a workspace that references local checkouts, and I have verified end to end against the real copilot-language-server (device-flow sign-in and inline ghost text that accepts with Tab). The package pins for CodeEditTextView / CodeEditSourceEditor need bumping to the releases that include layers 1 and 2 before this can build in CI. Would the maintainers welcome native Copilot support in CodeEdit?

Screenshots

I will attach a screen recording of the device-flow sign-in and inline ghost text on the PR.