feat: add one-time shell integration hint for new users

[avihut]

Summary

• Adds a one-time hint system that notifies users about shell integration when they run worktree commands without the shell wrapper active
• Hint is shown after successful worktree creation (clone, init, checkout, checkout-branch)
• Tracks which hints have been shown in ~/.config/daft/hints.json
• Can be suppressed via DAFT_NO_HINTS=1 environment variable

Problem

After installing daft from Homebrew, users may miss the shell integration setup instructions. Without shell integration, commands like git worktree-checkout-branch work but don't auto-cd into the new worktree - leaving users in their original directory wondering why.

Solution

After successful worktree operations, if the shell wrapper isn't active (DAFT_SHELL_WRAPPER env var not set), show a helpful one-time hint:

hint: Enable shell integration to auto-cd into new worktrees:
  eval "$(daft shell-init bash)"   # Add to ~/.bashrc or ~/.zshrc
  daft shell-init fish | source    # Add to ~/.config/fish/config.fish
Run 'daft shell-init --help' for more options.
To suppress hints: export DAFT_NO_HINTS=1

Test plan

• Unit tests for hint state management (save/load/mark shown)
• All existing tests pass
• Clippy passes with no warnings
• Code is properly formatted

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