[autoantohaki]

Requested by mau

Before: a tool that wanted people to change its settings after setup had to send them to a text editor, with no validation and no protection against a half-written file.

After: editConfig('./service.json', fields) shows every setting as a row (current value, (default), not set, or (invalid) for a stored value that no longer fits), edits one at a time with the same prompts and validation as a setup flow, marks unsaved changes, and writes the file only on s. q asks before discarding changes. npm run demo -- config shows it on an in-memory sample.

Targets main. Until #4 merges, the diff also shows the wizard and prompt fix from #4; merge #4 first and this PR shows only the configuration editor.

How: fields use dotted names (server.port) read and written through getValue/setValue/unsetValue, which reject __proto__-style keys. Keys no field describes are preserved. diffValues reports {name, before, after} per changed field. saveConfig writes two-space JSON to a temp file beside the target and renames it into place, keeping the replaced file's permissions unless mode is given (e.g. 0o600 for secrets). loadConfig treats a missing or empty file as {} and refuses non-object JSON instead of overwriting it. Guide in docs/config-editor.md.