feat(cli,format): deterministic JSON envelope with plumb_version, run_id, summary #98

Job	Run time
Preflight (fmt, clippy, check, agents-md)	38s
Determinism	49s
cargo-deny	35s
MSRV (1.95)	24s
Docs (build, no deploy)	39s
Size guard (<25 MiB)	2m 5s
Test (macos-latest)	1m 39s
Test (ubuntu-latest)	1m 26s
Test (windows-latest)	2m 57s
Coverage	1m 30s
	12m 42s

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