Fix _guess_type regression: posterior_fossa_t1 incorrectly classified as T1c

- Use regex word boundary matching to prevent 't1c' from matching 't1' in 'posterior_fossa_t1'
- Add 't1' to T1 patterns list to ensure it matches
- This fixes the regression where 'posterior_fossa_t1' was being classified as T1c instead of T1

Author: ruisoares1