Fix malformed copilot-cli feature reference in devcontainer.json

[Copilot]

The copilot-cli feature was pinned with an empty version tag (copilot-cli:), causing every devcontainer build to fail at compile time:

error: compile devcontainer.json: parse feature ref ghcr.io/devcontainers/features/copilot-cli:: could not parse reference: ghcr.io/devcontainers/features/copilot-cli:

A bad version bump (becd353) stripped the tag but left the dangling colon. Because the malformed ref blocks the whole file from compiling, it broke envbuilder, the devcontainer build CI step, and local VS Code builds.

Change

• Pin the feature to its major version, matching the rest of the file (github-cli:1, go:1):

- "ghcr.io/devcontainers/features/copilot-cli:": {},
+ "ghcr.io/devcontainers/features/copilot-cli:1": {},

This is the only malformed reference; no other features are affected.