Design: make split local OpenSpec libraries usable across the CLI #2011

Follow-up to #2008 and #2009: descendant discovery in list exposes work that view, show, validate, and change/workflow commands still cannot address from the repository root. Propose consistent local-library discovery and selection across all library-aware commands, with aggregated browsing and bulk validation, owning-library item lookup, explicit ambiguity diagnostics, and one-library scope for mutations. Registered stores remain explicitly scoped. A proposal-only PR will define the command-by-command contract before implementation.