chore(sonar): address all 97 code smells (5.5 lane)

Per-rule fixes:
- S1192 (string-literal dup → const): 83
- S3776 (cognitive complexity → split helpers): 7
- S1186 (empty function literals): 5
- S4144 (duplicated implementations): 1
- yaml:DocumentStartCheck: 1

vet/test/golangci-lint clean. No exported API changed.

Author: Snider