ci: modernize Go tooling and workflows

[madflojo]

Summary

Modernizes the project tooling and CI while keeping the existing Coveralls reporting. This also adds local plan ignores so planning files stay out of future diffs. Cleaner workshop, fewer loose nails.

Changes

• Add .plans/ to .gitignore for local planning notes.
• Update go.mod to Go 1.24.
• Expand Makefile targets for build, tests, lint, coverage, format, clean, benchmarks, and race benchmarks.
• Refresh test and lint workflows with current pinned actions and go-version-file.
• Keep Coveralls for coverage publishing.
• Add a benchmark workflow using benchmark-action/github-action-benchmark for non-race benchmarks.
• Run race-enabled benchmarks as part of the standard tests workflow.

Validation

• make build
• make tests
• make lint
• make coverage
• make benchmarks
• make benchmarks-race

Risks

• CI now requires Go 1.24.
• Benchmark tracking depends on the benchmark action cache path being available across workflow runs.