feat: add --out-json flag for structured report output

[nanookclaw]

Closes #5.

Adds an optional --out-json <path> flag to pgcompare run. When omitted, behavior is unchanged. When provided, the full ReportData is written as pretty-printed JSON next to the existing HTML report.

Changes

• main.go: new --out-json cobra flag; if set, call pgcompare.GenerateJSON(data, flagOutJSON) after the existing Generate call and print the JSON path to stdout alongside the HTML path.
• internal/pgcompare/report.go: add GenerateJSON(data ReportData, outPath string) error using encoding/json with MarshalIndent (2-space indent, 0644).
• internal/pgcompare/report_test.go: add TestGenerateJSON that round-trips a small ReportData and asserts GeneratedAt, Iterations, Speedups, Before.Stats[0].QueryName, and Diffs[0].QueryName are present in the JSON.

Stats.{Min,Max,P50,P95,P99,Mean,StdDev} are time.Duration, which encoding/json serializes as int64 nanoseconds — matches the structured-automation use case described in the issue.

Verification

go build ./...
go test ./internal/pgcompare/...   # 22 tests pass, incl. new TestGenerateJSON
gofmt -l .                          # empty
go vet ./...                        # clean

Default behavior unchanged: running pgcompare run --config ... --out report.html produces only report.html.