Add Context Store Foundation Tasks

• Research existing config, registry, file-system, and schema/backend patterns.
• Decide to start with Git/local backend identity only, not a generic file API.
• Decide that real context store roots are user-chosen Git checkouts or synced folders.
• Decide that the local registry lives under global data and portable store metadata lives inside the store root.
• Add context-store foundation types, path helpers, parse/serialize, and read/write helpers.
• Add focused tests for validation, paths, registry roundtrip, metadata roundtrip, and Git/local backend path resolution.
• Run targeted verification.
• Decide registration/resolution facade should precede initiative CLI.
• Add context-store registration/list/resolve facade and tests.