Lint: re-enable errcheck (and revive/gocritic/etc.) and clear pre-existing findings

[garyblankenship]

For the v0.1.0 release the .golangci.yml gate was reduced to a high-signal, green set. The following linters were disabled because the codebase has pre-existing findings:

• errcheck — ~48 unchecked errors across the codebase
• revive — ~80 exported-symbol doc-comment nits
• gocritic, gocyclo, errorlint, copyloopvar — a handful of findings each

# .golangci.yml — re-enable these under linters.enable:
#   - errcheck
#   - revive
#   - gocritic
#   - gocyclo
#   - errorlint
#   - copyloopvar

Reproduce the current debt: temporarily add them back and run golangci-lint run ./..., then clear the reported findings (errcheck first — highest signal) and re-enable each linter.