Add --preview flag to apply command for reviewing changes before applying

[that-github-user]

Summary

thinktank apply currently applies changes without showing them first. Users should be able to preview before committing.

Proposed

# Preview the recommended agent's diff
thinktank apply --preview

# Preview a specific agent
thinktank apply --preview --agent 2

Shows the full diff with syntax highlighting (if terminal supports it), then prompts "Apply these changes? [y/N]".

Also consider

• thinktank apply --dry-run — show what would change without prompting
• Pager support for large diffs (pipe through less or similar)

[that-github-user]

Fixed in #47. thinktank apply --preview shows the diff with syntax highlighting (green additions, red removals, cyan hunk headers) plus file list and change summary. Does not modify the working tree. Suggests the apply command after review.