revert: undo PR checkout fork support and unique branch naming

Reverts the following commits:
- f669191 fix: use unique local branch names for PR checkout to avoid conflicts (#931)
- 21e3fe0 Fix PR checkout to support fork PRs (#851)

Simplifies PR branch checkout back to using headRefName directly instead
of the pr-{number} local branch naming scheme introduced in #931 and the
GitHub pull ref fetch approach introduced in #851.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: ashwin-ant

src/github/operations/branch.ts

@@ -151,10 +151,7 @@ export async function setupBranch(
       // Handle open PR: Checkout the PR branch
       console.log("This is an open PR, checking out PR branch...");
 
-      const originalBranchName = prData.headRefName;
-      // Use unique local branch name to avoid conflicts with existing branches
-      // Format: pr-{number} ensures uniqueness since PR numbers are unique per repo
-      const localBranchName = `pr-${entityNumber}`;
+      const branchName = prData.headRefName;
 
       // Determine optimal fetch depth based on PR commit count, with a minimum of 20
       const commitCount = prData.commits.totalCount;
@@ -163,36 +160,24 @@ export async function setupBranch(
       console.log(
         `PR #${entityNumber}: ${commitCount} commits, using fetch depth ${fetchDepth}`,
       );
-      console.log(
-        `Fetching PR branch '${originalBranchName}' as local branch '${localBranchName}'`,
-      );
 
       // Validate branch names before use to prevent command injection
-      validateBranchName(localBranchName);
+      validateBranchName(branchName);
 
       // Execute git commands to checkout PR branch (dynamic depth based on PR size)
       // Using execFileSync instead of shell template literals for security
-      // Use GitHub's PR ref which works for both same-repo and fork PRs
-      // The local branch name uses pr-{number} format to ensure uniqueness
-      execGit([
-        "fetch",
-        "origin",
-        `--depth=${fetchDepth}`,
-        `pull/${entityNumber}/head:${localBranchName}`,
-      ]);
-      execGit(["checkout", localBranchName, "--"]);
+      execGit(["fetch", "origin", `--depth=${fetchDepth}`, branchName]);
+      execGit(["checkout", branchName, "--"]);
 
-      console.log(
-        `Successfully checked out PR #${entityNumber} as local branch '${localBranchName}'`,
-      );
+      console.log(`Successfully checked out PR branch for PR #${entityNumber}`);
 
       // For open PRs, we need to get the base branch of the PR
       const baseBranch = prData.baseRefName;
       validateBranchName(baseBranch);
 
       return {
         baseBranch,
-        currentBranch: localBranchName,
+        currentBranch: branchName,
       };
     }
   }