errcheck flagged six unchecked Close() calls. The production write
path in indexgen.ProcessDir now closes the index file explicitly
after Execute and surfaces the close error so a deferred flush
failure isn't lost; tests use _ = f.Close() since they don't care.
Also adds a 'make fixstyle' target that runs gofmt -w over the tree,
matching the convention used elsewhere.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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