fix(#1526): auto-mode worktree commits land on main instead of milestone branch (#1534)

GitServiceImpl.getMainBranch() was designed to detect manual /worktree worktrees
(worktree/<name> branches) but incorrectly applied the same logic to auto-mode
worktrees (milestone/<MID> branches). When no worktree/<name> branch existed,
it fell back to the current branch, which in certain contexts could be main,
causing slice commits to land on main instead of the milestone branch.

Fix: Detect if currently on a milestone/* branch first (auto-mode case) and
return it, before checking for worktree/* branches (manual worktree case).

- Modify getMainBranch() to detect milestone branches first
- Add test verifying getMainBranch() returns correct branch in auto-worktree
- All tests pass, build succeeds

Fixes #1526

src/resources/extensions/gsd/git-service.ts

@@ -479,9 +479,20 @@ export class GitServiceImpl {
 
     const wtName = detectWorktreeName(this.basePath);
     if (wtName) {
+      // Auto-mode worktrees use milestone/<MID> branches (wtName = milestone ID)
+      const milestoneBranch = `milestone/${wtName}`;
+      const currentBranch = nativeGetCurrentBranch(this.basePath);
+
+      // If we're on a milestone/<MID> branch, use it (auto-mode case)
+      if (currentBranch.startsWith("milestone/")) {
+        return currentBranch;
+      }
+
+      // Otherwise check for manual worktree branch (worktree/<name>)
       const wtBranch = `worktree/${wtName}`;
       if (nativeBranchExists(this.basePath, wtBranch)) return wtBranch;
-      return nativeGetCurrentBranch(this.basePath);
+
+      return currentBranch;
     }
 
     // Repo-level default detection: origin/HEAD → main → master → current branch.

src/resources/extensions/gsd/tests/auto-worktree.test.ts

@@ -153,6 +153,25 @@ async function main(): Promise<void> {
     // After teardown, originalBase should be null
     assertEq(getAutoWorktreeOriginalBase(), null, "no split-brain: originalBase cleared");
 
+    // ─── #1526: getMainBranch returns milestone branch in auto-worktree ──
+    console.log("\n=== #1526: getMainBranch() returns milestone/<MID> in auto-worktree ===");
+    {
+      const { GitServiceImpl } = await import("../git-service.ts");
+
+      // Create worktree
+      const wtPath = createAutoWorktree(tempDir, "M005");
+      // Don't set main_branch pref so getMainBranch falls through to worktree detection
+      const gitService = new GitServiceImpl(wtPath);
+      gitService.setMilestoneId("M005");
+
+      // Verify getMainBranch returns the milestone branch
+      const mainBranch = gitService.getMainBranch();
+      assertEq(mainBranch, "milestone/M005", "getMainBranch returns milestone/<MID> in auto-worktree");
+
+      // Cleanup
+      teardownAutoWorktree(tempDir, "M005");
+    }
+
     // ─── #778: reconcile plan checkboxes on re-attach ─────────────────
     console.log("\n=== #778: reconcile plan checkboxes on re-attach ===");
     {