prop-flow: make output batch-safe for a loop over changed files

[fxOne]

Summary

Makes prop-flow safe to run in a loop over changed files. A file with nothing to analyse is now a success rather than a failure, a handled failure under --json stays inside the JSON stream, and hooks are no longer mistaken for components. All three change the observable contract, so the package goes to 3.0.0.

Changes

• A file with no exported component is no longer an error. analyseProps returns an empty report and the CLI exits 0. Exit 2 is now reserved for what actually blocked the analysis: a missing file, no resolvable compiler config, or a tsconfig that does not span the file.
• Report gains a required components field — the number of exported components with a typed props object. It is what tells the two empty reports apart: no component to look at (0) versus a component whose props are all required.
• Under --json a handled failure is written to stdout as {"error": …} instead of to stderr, so an invocation that names a file emits exactly one JSON object whatever happens and a failing file no longer breaks the parse for the rest of the batch. The flag is read off the raw argv so a bad argument reaches the envelope too. Usage output (--help, and exit 1 with no file) deliberately stays plain text.
• An exported useX taking an options object is dropped from component discovery. It is indistinguishable from a component to the AST and has no JSX call sites, so every one of its options came back unused-component — a statement about the walk, not about the hook. Only discovery is narrowed; a prop passing through a hook is still traced and still reported at the component that declares it.
• Text output keeps its tsconfig and Program file-count header on an empty report, and says "no exported component" where that is the reason rather than "no props to report".
• formatJsonError is exported from the package index; resolveTarget is extracted out of analyseProps to separate target resolution from the prop walk.
• README documents the batching workflow and the exit-code contract; CHANGELOG and version updated for 3.0.0.