Add Experimental field to CommandInfo (omitempty JSON) and mark the tui
entry. The `basecamp commands` output prefixes experimental descriptions
with [experimental]; the JSON response includes the boolean field.

Cobra Short/Long get the [experimental] tag and active-development
disclaimer. An annotation on the command provides a hook for future
generic detection.

A one-time-per-version stderr advisory prints on first `basecamp tui`
invocation, gated by a sentinel file in CacheDir. Resets on upgrade so
the notice resurfaces when experimental behavior is most likely to have
changed.

Add experimentalBadge field and SetExperimental setter to Breadcrumb.
When enabled, renders an [experimental] badge in Warning color before
the account badge.

Wire SetExperimental(true) in Workspace.New() — hardcoded since the
TUI is the only workspace entry point. Promotion to stable is just
removing the call.