feat: detect when multiple windows open the same workspace

[EhabY]

Summary

Adds WindowBroadcast<T>, a generic typed pub/sub channel built on top of SecretStorage.onDidChange, and uses it to detect when more than one VS Code window is connected to the same Coder workspace.

Closes #783, #548

How it works

When a user clicks Connect, the extension fires openFolder() immediately and sends a PING through SecretStorage in parallel. If another window responds with a PONG, the clicking window shows a non-blocking notification with two choices:

• Duplicate Window: tells the responding window to run duplicateWorkspaceInNewWindow, preserving its tabs, panels, and remote connection.
• Open Without Folder: opens a new window connected to the remote with no folder.

The whole chain is fire-and-forget. openWorkspace() never blocks on it, so if no other window is listening the behavior is identical to before.

Layout

File	Role
src/ipc/windowBroadcast.ts	Generic typed pub/sub primitive over a single SecretStorage key
src/workspace/duplicateWorkspaceIpc.ts	PING / PONG / DUPLICATE protocol used by the open-workspace flow
src/oauth/oauthCallback.ts	Cross-window OAuth redirect signal, also built on WindowBroadcast
src/commands.ts	Sends PING in parallel with openFolder() and shows the notification
src/extension.ts	Responds to PINGs and runs duplicateWorkspaceInNewWindow when targeted

Each consumer of WindowBroadcast defines its message shape with a Zod schema, so invalid or stale data on the storage key is dropped before reaching handlers.

Notes

• WindowBroadcast is the only generic piece. Specializations live with their feature (OAuth signal in oauth/, workspace duplicate detection in workspace/) rather than in a shared IPC bucket.
• The notification uses non-modal showInformationMessage, which VS Code may dismiss without resolving, so the chain must not be awaited from the open path.
• The notification is shown in the window where the user clicked, so it always appears in a predictable place regardless of which window VS Code routes focus to.