feat(vcs): Prefer upstream remote over origin for base repo name (#2737)

## Summary
Prefer "upstream" remote over "origin" when determining base repository
name, following Git fork conventions.

🤖 Generated with [Claude Code](https://claude.ai/code)

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: runningcode

src/utils/vcs.rs

@@ -284,7 +284,7 @@ fn find_merge_base_ref(
 }
 
 /// Attempts to get the base repository name from git remotes.
-/// Prefers "origin" remote if it exists, otherwise uses the first available remote.
+/// Prefers "upstream" remote if it exists, then "origin", otherwise uses the first available remote.
 /// Returns the base repository name if a remote is found.
 pub fn git_repo_base_repo_name(repo: &git2::Repository) -> Result<Option<String>> {
     let remotes = repo.remotes()?;
@@ -295,8 +295,10 @@ pub fn git_repo_base_repo_name(repo: &git2::Repository) -> Result<Option<String>
         return Ok(None);
     }
 
-    // Prefer "origin" remote if it exists, otherwise use the first one
-    let chosen_remote = if remote_names.contains(&"origin") {
+    // Prefer "upstream" if it exists, then "origin", otherwise use the first one
+    let chosen_remote = if remote_names.contains(&"upstream") {
+        "upstream"
+    } else if remote_names.contains(&"origin") {
         "origin"
     } else {
         remote_names[0]