fix(branch): collapse empty path segments in branch_name_template (#1539)

A branch_name_template that places {{description}} (or another variable)
next to a slash crashes the run when the variable resolves to an empty
string. An issue/PR title with no ASCII-alphanumeric content — emoji-only,
CJK-only, or punctuation-only — makes extractDescription() return "", so a
template like "{{prefix}}{{description}}/{{entityNumber}}" produces
"claude//123". validateBranchName rejects consecutive (and leading/trailing)
slashes, and the thrown error propagates uncaught out of setupBranch,
aborting the entire run.

Normalize the templated result before the empty-result check: collapse runs
of slashes and drop any leading/trailing slash. Single-slash and dash
separators are untouched, so existing template behavior is unchanged; a
template that collapses to empty still falls back to the default format.

This is distinct from the {{label}} sanitization tracked in #1491 (and its
open PRs), which deliberately leave {{description}} alone — so this path
remained broken. Fixes the whole empty-segment class regardless of variable.

Adds regression tests for emoji-only and CJK-only titles, a trailing empty
segment, and a direct validateBranchName assertion proving the run no longer
aborts.

test/branch-template.test.ts

@@ -291,5 +291,68 @@ describe("branch template utilities", () => {
       expect(result).toMatch(/^fix\/pr-456-\d{8}-\d{4}$/);
       expect(result.length).toBeLessThanOrEqual(50);
     });
+
+    // Regression: a title with no ASCII-alphanumeric content makes
+    // {{description}} sanitize to an empty string. Around a slash separator this
+    // previously produced "claude//123", which validateBranchName rejects
+    // ("cannot contain consecutive slashes"), aborting the entire run.
+    it("should collapse the double slash from an empty description (emoji-only title)", () => {
+      const template = "{{prefix}}{{description}}/{{entityNumber}}";
+      const result = generateBranchName(
+        template,
+        "claude/",
+        "issue",
+        123,
+        undefined,
+        undefined,
+        "🎉🎉🎉",
+      );
+
+      expect(result).toBe("claude/123");
+    });
+
+    it("should collapse the double slash for a CJK-only title", () => {
+      const template = "{{prefix}}{{description}}/{{entityNumber}}";
+      const result = generateBranchName(
+        template,
+        "claude/",
+        "issue",
+        123,
+        undefined,
+        undefined,
+        "日本語のタイトル",
+      );
+
+      expect(result).toBe("claude/123");
+    });
+
+    it("should drop a trailing slash left by an empty trailing segment", () => {
+      const template = "{{prefix}}{{entityNumber}}/{{description}}";
+      const result = generateBranchName(
+        template,
+        "claude/",
+        "issue",
+        123,
+        undefined,
+        undefined,
+        "!!! ???",
+      );
+
+      expect(result).toBe("claude/123");
+    });
+
+    it("should produce a name that passes validateBranchName when a segment is empty", () => {
+      const result = generateBranchName(
+        "{{prefix}}{{description}}/{{entityNumber}}",
+        "claude/",
+        "issue",
+        123,
+        undefined,
+        undefined,
+        "🎉",
+      );
+
+      expect(() => validateBranchName(result)).not.toThrow();
+    });
   });
 });