fix(cli): resolve all five HIGH UX issues from CLI audit

[deveshctl]

Summary

• HIGH-1/docs: Remove broken --platform per-argument compare example from README and migrating-from-dive.md — --platform is a single-value flag and the example implied per-image pinning that doesn't exist
• HIGH-2/docs: Fix jq examples in README that referenced nonexistent snake_case fields (wasted_bytes) — replaced with correct camelCase field paths (efficiency.score, layers[].size, efficiency.wastedFiles[].totalWasted)
• HIGH-3/fix: Demote --json from PersistentFlags to local Flags on rootCmd and ciCmd; compareCmd never registers it so passing --json compare now fails at parse time with "unknown flag" instead of at runtime — adds TestCompareCmd_JSONFlag_UnknownAtParseTime
• HIGH-4/fix: completeImageRefs now calls engineBinaryForCompletion(engineFlag) instead of hardcoding docker, so Podman users get correct tab completions — adds cmd/completion_test.go
• HIGH-5/fix: Add warnCIThresholdFlagsIgnored helper that scans raw os.Args and emits a warning: to stderr when CI=true is active and a threshold flag name is present; also documents the limitation in the root --help Long description

Note: HIGH-3 Part B (JSON output on compare) is a new feature and is deferred to a separate branch. All five HIGH items are marked ✓ DONE in internal-docs/Revamp_cli.md.

Test Plan

• go test ./cmd/... — all existing tests pass; new tests: TestCompareCmd_JSONFlag_UnknownAtParseTime, TestCompletionEngineBinary_*, TestCompleteImageRefs_*, TestWarnCIThresholdFlagsIgnored_*
• layerx compare --json out.json img:a img:b → "unknown flag: --json" at parse time (not "not supported")
• layerx --engine podman completion bash — tab completions query podman images instead of docker images
• CI=true layerx --help — Long description mentions threshold flag limitation