Add --diff-only flag to show just the diff without full output

[that-github-user]

Summary

Power users piping thinktank output often just want the diff. A --diff-only flag on run/apply would output only the recommended agent's unified diff to stdout, enabling:
```bash
thinktank run "fix bug" -n 3 --diff-only | git apply
thinktank run "fix bug" -n 3 --diff-only > fix.patch
```
This makes thinktank composable with other tools.

[that-github-user]

Fixed in #149. --output-format diff outputs just the recommended agent's unified diff. Enables piping: thinktank run 'fix' --output-format diff | git apply