feat: add standalone Checkstyle, dotenv, and Kubernetes linting

[zeitlinger]

Summary

• add standalone Checkstyle support using the curated JAR and a Java runtime on PATH
• add safe, explicit-path dotenv-linter checking and fixing
• add report-only kube-linter support for selected Kubernetes manifests while excluding Compose and unrelated YAML
• add tool pins, Renovate metadata, generated docs, and focused end-to-end cases

This PR is stacked on the Flint v2 foundation PR and does not change a consumer repository yet.

Validation

• cargo fmt --all -- --check
• mise exec -- cargo clippy --all-targets -- -D warnings
• mise exec -- cargo test --bin flint (293 passed; local test used a temporary no-op dotnet executable because the environment does not provide a working dotnet binary)
• FLINT_CASES=checkstyle cargo test -q --test e2e cases
• FLINT_CASES=dotenv-linter cargo test -q --test e2e cases
• FLINT_CASES=kube-linter cargo test -q --test e2e cases
• mise run generate
• mise run lint:fix
• mise run validate-mise-registry

[zeitlinger]

Superseded by the reorganized capability stack in #430.