fix(branch): collapse empty path segments in branch_name_template

[NickNojiri]

Summary

Fixes #1527. 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.

This is distinct from the {{label}} sanitization tracked in #1491 (and its
PRs #1492 / #1498), which deliberately leave {{description}} alone — so this
path remains broken regardless of which of those merged. This fixes the whole
empty-segment class, independent of which template variable is empty.

Fix

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.

Changes

• src/utils/branch-template.ts — collapseEmptyPathSegments() applied on
  the custom-template path
• test/branch-template.test.ts — 4 regression tests: emoji-only title,
  CJK-only title, trailing empty segment, and a direct validateBranchName
  "does not throw" assertion proving the run no longer aborts

Testing

• bun test — 774 pass, 0 fail
• bun run typecheck — clean
• bun run format:check — clean