fix: allow , in branch names (#1310)

`validateBranchName` rejects branch names containing a comma, even
though `git check-ref-format` permits commas and GitHub itself accepts
them. PRs whose head branch contains a `,` fail validation in-process
before any git operation, so the action errors out immediately.

Branch names with commas show up in real workflows when names are
derived from titles, place names, or external identifiers (e.g.
"feature/paris,france"). There is no workaround other than renaming
the branch, which is often not under the user's control.

All git calls in this file use execFileSync with an argv array, so no
shell interpretation occurs and `,` carries no injection risk. This is
the same reasoning used to add `#` in #1167 and `+` in #1248.

- Add `,` to the validateBranchName whitelist regex
- Update the surrounding comment and error message to match
- Add a test case covering commas in title-derived branch names

Fixes #1300

test/validate-branch-name.test.ts

@@ -55,6 +55,15 @@ describe("validateBranchName", () => {
       expect(() => validateBranchName("fix+issue-123")).not.toThrow();
       expect(() => validateBranchName("feature+new-thing")).not.toThrow();
     });
+
+    it("should accept branch names containing , (git-valid, common in title-derived branches)", () => {
+      // Reported in #1300: branches like "feature/a,b" were rejected, even though
+      // git check-ref-format and GitHub both accept commas. Common when branch names
+      // are derived from titles, place names, or external identifiers.
+      expect(() => validateBranchName("feature/a,b")).not.toThrow();
+      expect(() => validateBranchName("feature/paris,france")).not.toThrow();
+      expect(() => validateBranchName("fix/issue-1,2,3")).not.toThrow();
+    });
   });
 
   describe("command injection attempts", () => {