Add server-side branch prefix search

[HarshMN2345]

Uses GraphQL refs query with query variable for prefix filtering and cursor-based pagination instead of fetching all branches client-side.

[greptile-apps]

Greptile Summary

This PR replaces the REST-based listBranches implementation with a GitHub GraphQL call that accepts a $search parameter and passes it as the query argument on the refs connection. A defensive guard for null author fields in getLatestCommit is also included.

• listBranches now issues a single GraphQL request using refs(query: $search) for server-side filtering, returns a flat array<string>, and clamps $perPage to [1, 100]; the $page parameter is accepted but not used in the new implementation.
• The test suite drops the old page-1/page-2 pagination assertions and adds search-filter assertions, including one that asserts a non-prefix substring ('ranch') returns results — which conflicts with the PR title ("prefix search") and the docblock ("Substring filter"), leaving the intended matching semantics ambiguous.

Confidence Score: 4/5

The core implementation is functionally sound for basic search use cases, but the accepted $page parameter is silently ignored and the test makes assumptions about GitHub API matching semantics that contradict the PR's stated purpose.

The GraphQL query correctly fetches branches with optional search filtering and returns a flat array. However, the $page parameter is accepted in the signature but never read — callers expecting paged navigation will silently receive only the first page. The new test also asserts that a non-prefix substring ('ranch') returns results while the PR title and docblock are internally inconsistent about whether the feature is prefix or substring matching, making it unclear which behavior is intentional and whether the test will pass against the live API.

Both src/VCS/Adapter/Git/GitHub.php and tests/VCS/Adapter/GitHubTest.php warrant a closer look — the implementation ignores the $page argument and the test's matching-semantics assertion may not hold against the real GitHub API.

Important Files Changed

Filename	Overview
src/VCS/Adapter/Git/GitHub.php	Replaces REST-based listBranches with a GraphQL implementation supporting server-side search; also defensively guards against null author in getLatestCommit. The $page parameter is accepted but unused in the new GraphQL path, and the $search docblock says "Substring filter" while the PR title says "prefix search".
tests/VCS/Adapter/GitHubTest.php	Removes old page-1/page-2 pagination tests and adds search-filter tests; a new assertion expects 'ranch' (a substring, not a prefix) to match 'branch-a'/'branch-b', which contradicts the PR title ("prefix search") and may fail against the real API.

_{Reviews (9): Last reviewed commit: "revert pagination" | Re-trigger Greptile}

[Meldiron]

Keep old response structure - no hasNext, no cursor

[Meldiron]

Lets not loop

[Meldiron]

Lets avoid helper method