Make the Git HTTP server instance-based and storage-backed

[asim]

This starts the refactor toward a small embeddable Git server without changing the current command-line usage.

What changes

• adds a Server type implementing http.Handler
• moves request handling onto instance configuration rather than relying directly on global state
• introduces Repository and Store
• adds the default filesystem-backed store
• keeps server.Handler() working for compatibility
• adds basic filesystem store tests

Why

This creates the first clean abstraction boundary needed for pluggable repository storage and additional transports later, while continuing to use the native Git binary as the Git engine.

Follow-up

A next PR can expand Store with repository lifecycle operations (Create, Delete, List) and then add authorization/hooks independently of HTTP.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-06T05:12:43.678300Z	52fce60	PR opened

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Reviewed commit: 52fce605ae

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