isLikelyHash() required at least one digit to distinguish hashes from
English words, but base36 hashes can be all-letters by chance.
This caused ExtractIssuePrefix("xa-adt-bat") to return "xa" instead
of "xa-adt", breaking import for 20 issues in xa-adapt.
Fix: Accept all-letter suffixes for 3-char only, keep digit requirement
for 4+ chars where word collision probability is low enough (~0.2%).
Rationale:
- 3-char: 36³ = 46K hashes, ~1000 common words = ~2% collision
- 4-char: 36⁴ = 1.6M hashes, ~3000 words = ~0.2% collision
- 5+ char: collision rate negligible
d541ff48e3