[Initiative] API driven repository creation

[juliuskrah]

Summary

Starting GitStore currently requires running an initialization script to bootstrap the backing Git repository. This is not end-user friendly. Implement API-driven repository creation so the platform can provision repositories programmatically, similar to Git hosting platforms that create repositories via API calls.

Scope

In scope:

• Add API capabilities to create and initialize repositories without requiring manual init scripts.
• Implement server-side bootstrap flow in gitstore-git-service for repository initialization and default branch setup. (Implemented in [Initiative] Migrate gitstore-git-service from git2 to gitoxide #108)
• Expose orchestration path from gitstore-api to request repository creation through service APIs.
• Enforce namespace-scoped repository creation (<namespace>/<repo>) so repositories cannot be created at root scope.
• Integrate with API-driven namespace creation/lifecycle once provided by [Initiative] Namespaces #39.
• Return clear error handling for already-existing repos, invalid inputs, and permission failures.
• Update docs to describe API-first bootstrap flows for users and operators.

Out of scope:

• Defining namespace domain model and cross-namespace policy behavior (covered by [Initiative] Namespaces #39).
• Building a full GitHub-equivalent repository management product surface in this initiative.
• Removing existing scripts before API-driven bootstrap reaches parity and is validated.

Acceptance Criteria

• A repository can be created and initialized through API flows without running init scripts manually.
• Repository creation requires an existing namespace and rejects root-scope creation.
• API responses clearly indicate success/failure and include actionable errors.
• Bootstrap flow supports namespace-aware repository creation once namespaces are available.
• Documentation includes API-first setup path and migration notes from script-based initialization.

Subtasks

• [Initiative] API-Driven Bare Repository Provisioning #111

Tracking

• Dependency: Blocked by [Initiative] Namespaces #39 (Namespaces, including API-driven namespace creation), [Initiative] Decouple API from Git Storage via gRPC Git Service #65
• Milestone: TBD