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 GraphQL query that supports prefix-filtered search via the refs(query:) argument and cursor-based pagination for pages beyond the first. It also makes author fields optional in getLatestCommit to handle commits whose GitHub account has been removed.

• listBranches now issues a GraphQL refs query with an optional query variable for server-side prefix filtering; a separate cursor-preflight query is used when $page > 1 to derive the correct after cursor.
• getLatestCommit removes the hard requirement for avatar_url and html_url on the author object, returning empty strings when those fields are absent instead of throwing.

Confidence Score: 3/5

Two defects in the changed code need fixing before this is safe to merge: the cursor preflight query breaks for pages 4+ and the new substring test will fail against the real API.

The cursor query passes ($page - 1) * $perPage as first; with $perPage capped at 50 this exceeds GitHub GraphQL's hard limit of 100 starting at page 4, silently returning empty results. Additionally, the new integration test asserts that refs(query: 'ranch') returns branches containing the string 'ranch', but GitHub's refs(query:) is prefix-only — the test will fail against the real API on every run.

Both src/VCS/Adapter/Git/GitHub.php (cursor first limit) and tests/VCS/Adapter/GitHubTest.php (incorrect substring assertion) need attention before merging.

Important Files Changed

Filename	Overview
src/VCS/Adapter/Git/GitHub.php	Rewrites listBranches to use GraphQL with cursor-based pagination and prefix search; also makes author fields optional in getLatestCommit. The cursor query's first variable can exceed GitHub's 100-item limit for pages 4+, silently returning empty results. Docblock incorrectly states the perPage ceiling as 100 instead of 50.
tests/VCS/Adapter/GitHubTest.php	Adds search assertions to testListBranchesPagination; the new $substringSearch assertion incorrectly expects prefix-only refs(query: 'ranch') to return branch names containing 'ranch', which will fail against the real GitHub API.

_{Reviews (12): Last reviewed commit: "cap perPage at 50 to avoid GraphQL 100-i..." | Re-trigger Greptile}

[Meldiron]

Keep old response structure - no hasNext, no cursor

[Meldiron]

Lets not loop

[Meldiron]

Lets avoid helper method

[greptile-apps]

Cursor query first value exceeds GitHub's GraphQL limit for pages 4+

The cursor query passes ($page - 1) * $perPage as first. With $perPage clamped to a maximum of 50, $page = 4 yields first = 150, which exceeds GitHub GraphQL's hard limit of 100 for any connection field. GitHub returns a validation error; the code at line 783 treats a non-array response as "no next page" and returns [], so every request for page 4 or higher silently returns an empty result instead of the actual data.