Add GitHub Copilot CLI to the development container

[chalew101]

Issue Description

Problem

The development container does not install GitHub Copilot CLI as part of its
declared features. Contributors must install it manually, and the CLI may not
be available consistently after rebuilding the container.

Proposed Change

Add the official ghcr.io/devcontainers/features/copilot-cli:1 feature to the
development container and commit the generated feature lock entry.

Update the devcontainer documentation to list Copilot CLI as an included tool.

Acceptance Criteria

• Copilot CLI is declared in .devcontainer/devcontainer.json.
• The feature is pinned in .devcontainer/devcontainer-lock.json.
• /usr/local/bin/copilot --version succeeds after rebuilding the container.
• The devcontainer documentation lists Copilot CLI.
• Existing devcontainer functionality remains unchanged.

Additional Context

No response