chore(code): hide environment pickers until environments are set up

[VojtechBartos]

Problem

The Workspace Mode dropdown and the Worktree environment picker both showed "add" / "+" affordances even when no environments had been configured yet, nudging users toward a feature they may not need. The Environments and Cloud environments settings screens also lacked a clear explanation of what these things are and how they're used.

Changes

Hide pickers until something is set up

• WorkspaceModeSelect: the + next to "Cloud environments" only shows once at least one cloud environment exists.
• EnvironmentSelector (worktree env picker in task input): renders nothing when the project has no environments defined, so it no longer takes up space with a dead dropdown.

Help text in settings

• Environments list and Cloud environments list: added a short paragraph explaining what the thing is, how it's used, and where it's picked.
• Environment form (local): per-field help for Name and Setup script (when it runs, what it's for).
• Cloud environment form: per-field help for Name, Network access (Full / Trusted / Custom), Allowed domains (wildcard syntax), "Include default domains" checkbox, and Environment variables (.env format, how editing works).

Showcase

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