Add Context Store Foundation Evidence

Research Summary

Existing OpenSpec patterns point toward a small explicit foundation:

Global data uses XDG/platform locations from getGlobalDataDir().
Workspace registries are machine-local convenience indexes under global data.
Workspace portable state uses versioned YAML and strict Zod validation.
Existing read/write helpers validate state before writing and use FileSystemUtils.writeFile() to create parent directories.
Schema/backend-style code favors small explicit adapters and registries over heavy framework abstractions.

The first context-store backend is Git/local checkout config only.
OpenSpec records where the local checkout lives; it does not decide where real team stores are cloned by default.
The local registry is not source of truth. It is a machine-local index.
Store-root metadata is portable source-of-identity for the synced store.
Initiatives and collections are later consumers, not part of the store foundation.
A thin facade should hide raw registry/metadata writes before initiative CLI wiring.

src/core/context-store/registry.ts registers Git/local context stores, lists local registry entries, and resolves registered stores with metadata id validation.
src/core/context-store/index.ts exports the facade.
test/core/context-store/registry.test.ts covers registration, registry merge/update, metadata mismatch rejection, listing, resolution, missing or mismatched metadata, and initiative collection mounting from a resolved root.