test: add e2e tests for portable shell completion syntax

[timvw]

Summary

Add e2e tests to verify:

• shellenv uses portable sed syntax (not gawk-specific match())
• The sed command correctly extracts branch names from worktree list

This prevents regression of the macOS awk compatibility issue fixed in #34.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 17.64%. Comparing base (96420f4) to head (5401501).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #35   +/-   ##
=======================================
  Coverage   17.64%   17.64%           
=======================================
  Files           3        3           
  Lines         958      958           
=======================================
  Hits          169      169           
  Misses        767      767           
  Partials       22       22           

see 1 file with indirect coverage changes

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