[Initiative] Decouple API from Git Storage via gRPC Git Service

[juliuskrah]

Summary

Replace filesystem-level coupling between gitstore-api and gitstore-git-service with a gRPC contract (Gitaly-inspired) so all Git read/write operations are executed by gitstore-git-service. The API must no longer depend on a shared Git directory or bundle a Git client library such as go-git. Existing websocket notifications continue to be used for push-based update signaling where appropriate.

Scope

In scope:

• Define and implement gRPC APIs in gitstore-git-service for Git read and write operations required by gitstore-api.
• Migrate gitstore-api Git operations to gRPC clients and remove direct repository access through shared volume assumptions.
• Remove API-side dependency on embedded Git client behavior for repository mutation/read paths (including eliminating go-git usage in API Git interaction paths).
• Preserve or adapt websocket-triggered update propagation so API cache/reload behavior still reacts to repository changes.
• Add integration and contract tests for API <-> git-service gRPC interactions covering both read and write flows.
• Update architecture and developer docs to reflect gRPC service boundaries and runtime configuration.

Out of scope:

• Replacing the KV-optimized read layer for latest tag/release or changing its purpose.
• Rewriting unrelated catalog query optimizations not tied to API/git-service transport decoupling.
• Introducing a full Git monolith replacement beyond the scoped gRPC service boundary.

Acceptance Criteria

• gitstore-api performs required Git reads/writes via gRPC to gitstore-git-service, not via shared Git directory access.
• API runtime no longer requires direct Git repository filesystem mounting for core Git operations.
• API Git operation paths no longer rely on bundled go-git behavior.
• gRPC read and write paths are covered by automated integration/contract tests in CI.
• Websocket-driven update signaling remains functional with the new gRPC-based interaction model.
• Documentation clearly distinguishes this transport/service decoupling work from KV latest tag/release read optimizations.

Tracking

• Milestone: TBD