Add merge, index batch operations, conflict handling, and remote URL …

…update APIs

- Add Repository.merge(branch:message:) with full merge flow (analysis, fast-forward, normal merge)
- Add MergeResult enum and ConflictEntry model for programmatic conflict resolution
- Add Repository.addAll() (git add -A) and updateAll() (git add -u) for batch staging
- Add conflict detection/iteration/removal APIs (hasConflicts, conflicts, removeConflict)
- Make index remove operations public (previously internal)
- Add RemoteCollection.setURL(named:to:) for updating remote URLs
- Add Repository.cleanupState() for post-merge state cleanup

Made-with: Cursor

Refactor initialization in SwiftGitXTest and SignatureRawTests to rem…

…ove async requirement to fix github action

Auto-create push refspecs and validate remote

Add createsRefspec parameter (default true). Ensure current branch and
remote are resolved, check for an existing push refspec and either add
one or error depending on createsRefspec. Introduce hasPushRefspec and
addPushRefspec helpers and use configured refspecs for git_remote_push.

Merge pull request ibrahimcetin#5 from ibrahimcetin/lazy-tree

Implement lazy loading for Commit.tree property

Format all codebase with swift-format

Update libgit2 version to 1.9.1

Merge pull request ibrahimcetin#2 from TimArt/main

Fix Swift Package Build

Add Foundation import to fix Xcode 16 errors

Conform Repository to Codable, Equatable and Hashable

Fix branch collection memory leak