Devtree 0.4.0: Global environment registry and `devtree list` command

Devtree now tracks every running development environment in a machine-wide registry, so you can see all active worktrees and their URLs from any directory with the new devtree list command. Sessions are created automatically when devtree dev starts, removed on clean exit, and pruned when stale processes are detected.

• New devtree list command: devtree list (or devtree list --json for machine-readable output) shows every running Devtree environment on the machine — including instance name, status, PIDs, public URL, and worktree path — from any directory.
• Global environment registry: Each devtree dev session registers itself in ~/.devtree/sessions/ with its worktree path, public URL, Devtree process ID, and dev-runner process ID. Sessions are removed on exit and stale entries from crashed processes are pruned automatically.
• registry.enabled configuration: A new registry section in .devtree.yml lets you opt a project out of the machine-wide list with registry: enabled: false. The setting only controls the live environment list — routing state and Docker dependency metadata are unaffected. Developers can override it in .devtree.local.yml.
• info shows registry status: The info command now reports whether the environment registry is enabled or disabled for the current checkout.
• Documentation updated: README and Devtree skills updated to cover the list command and registry configuration.