[WIP] Add POSIX CLI kv tool for parsing key-value pairs

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original prompt

Build POSIX CLI kv (shell + optional jq). Read stdin/file. Parse tokens: key=value, key="v", key='v', and header key: value. kv parse -> JSONL (1 obj/line). kv json -> single JSON (error if >1 line). Flags: --mode strict|loose, --delim auto|equals|colon, --trim, --keep-empty, --allow-dup (arrays). Add bin/kv, README examples (incl jsonl), MIT, basic shell tests. Open PR.

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