refactor: rename cd marker to user-friendly format

[timvw]

Summary

• Change output from TREE_ME_CD:/path to wt navigating to: /path
• More readable when shell integration captures the output

Before

✓ Worktree created at: /path/to/worktree
TREE_ME_CD:/path/to/worktree

After

✓ Worktree created at: /path/to/worktree
wt navigating to: /path/to/worktree

Test plan

• All unit tests pass
• Verified new output format works with shell integration

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 17.64%. Comparing base (9d61b93) to head (c61f430).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
main.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #33   +/-   ##
=======================================
  Coverage   17.64%   17.64%           
=======================================
  Files           3        3           
  Lines         958      958           
=======================================
  Hits          169      169           
  Misses        767      767           
  Partials       22       22           

Files with missing lines	Coverage Δ
main.go	19.52% <0.00%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.