fix: accept 3-char all-letter base36 hashes in ExtractIssuePrefix (#446)

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

internal/utils/id_parser_test.go

@@ -413,9 +413,9 @@ func TestExtractIssuePrefix(t *testing.T) {
 			expected: "my-cool-app", // 3-char base36 hash
 		},
 		{
-			name:     "3-char all-letters suffix (should fall back to first hyphen)",
+			name:     "3-char all-letters suffix (now treated as hash, GH #446)",
 			issueID:  "test-proj-abc",
-			expected: "test", // All letters = not a hash, falls back to first hyphen
+			expected: "test-proj", // 3-char all-letter now accepted as hash (GH #446)
 		},
 	}