Add repository lifecycle to Store

[asim]

This adds repository lifecycle operations to the storage abstraction without coupling storage to the Git executable.

Changes

• expands Store with Create, Delete, Exists, and List
• adds ErrRepositoryExists
• implements lifecycle operations for FilesystemStore
• supports nested repository names such as team/example.git
• rejects path traversal outside the store root
• returns sorted repository names from List
• adds lifecycle and interface-conformance tests

Design

Store.Create allocates repository storage but does not run git init. Git initialization belongs to the server/Git-engine layer so future remote or object-backed stores do not need to invoke the local Git binary.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-06T06:49:15.673067Z	fd46971	PR opened

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: fd46971208

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