fix: CI lint silently breaks whenever a new Go stable is released

[emyller]

Every pull request opened on this repository today fails the lint stage with errors unrelated to the change under review, because the bundled type-checker is built against the Go 1.24 standard library while the workflow runs on stable, which advanced to Go 1.26 with its February 2026 release. The break is pure toolchain drift; the source code is otherwise clean. Restoring CI requires moving the lint action to its v8+ line, which in turn requires migrating the lint configuration to the golangci-lint v2 schema. Going to v2 also drops the bundled suppressions v1 inherited by default, surfacing two findings the project had been silently hiding — both addressed inline rather than re-suppressed.

Changes

• Pull requests that touch no CI configuration pass the lint stage on the most recent released Go.
• Lint findings previously hidden by the toolchain's default suppressions are made explicit in source.
• Future automated bumps to the lint action land cleanly because the configuration matches current upstream conventions.

Closes #206

Review effort: 2/5