fix: show helpful error when running git gtr cd

[helizaga]

Summary

• git gtr cd now shows a clear error explaining why it doesn't work (subprocess limitation) instead of the generic "Unknown command" message
• Prints the exact eval "$(git gtr init <shell>)" setup command, auto-detecting the user's shell
• Guides users to use gtr cd (the shell function wrapper) instead

Before

$ git gtr cd
[x] Unknown command: cd
Use 'git gtr help' for available commands

After

$ git gtr cd
[x] 'cd' requires shell integration (subprocesses cannot change your shell's directory)
[i] Set up with: eval "$(git gtr init zsh)"
[i] Then use:    gtr cd [<branch>]

Test plan

• git gtr cd shows helpful error with shell-specific setup instructions
• gtr cd (shell wrapper) still works normally
• All 282 BATS tests pass

Summary by CodeRabbit

Release Notes

• Bug Fixes
  • Enhanced error handling for the "cd" command with improved guidance on shell integration setup and initialization instructions.

[coderabbitai]

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ℹ️ Recent review info

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Review profile: CHILL

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📥 Commits

Reviewing files that changed from the base of the PR and between daee0c1 and 405b647.

📒 Files selected for processing (1)

• bin/git-gtr

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Walkthrough

Adds a dedicated handler for the "cd" command within the main command dispatcher. When invoked, the handler determines the shell name, emits an error message indicating cd requires shell integration, and suggests initialization via git gtr init <shell> before exiting with status 1.

Changes

Cohort / File(s)	Summary
CD Command Handler bin/git-gtr	Added 8-line conditional handler for "cd" command that emits integration error message and provides initialization guidance before exiting.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Poem

🐰 A hop, a stop, a gentle guide,
When cd is called with nowhere to hide,
"Init your shell!" the rabbit cries with glee,
For some commands need integration's key.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: adding a helpful error message when running 'git gtr cd' instead of a generic unknown command message.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/cd-helpful-error

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