fix(cli): default to text format when --format is omitted

[dmchaledev]

Problem

The primary documented CLI invocation crashes:

$ npx @hailbytes/sbom-diff old.json new.json
Error: Unsupported format: old.json
    at renderReport (dist/reporter.js:11:24)
    at main (dist/cli.js:30:17)

This is the very first example in the README's Quick Start, so every new user following the docs hits it.

Root cause

In src/cli.ts, format parsing fell through to the positional form even when --format was absent:

const formatArg = args.find(a => a.startsWith('--format='))?.split('=')[1]
  ?? args[args.indexOf('--format') + 1];

When --format is not passed, args.indexOf('--format') returns -1, so args[-1 + 1] → args[0] → the old file path gets used as the format. renderReport() then throws on the unknown format.

Fix

• Extract a unit-tested resolveFormat() helper that supports both --format json and --format=json, and defaults to text when the flag is absent or empty.
• Validate the value and emit a clean one-line error (no stack trace) for unsupported formats: Unsupported format: xml. Valid formats: text, json, markdown.
• Guard main() so it only runs when the module is invoked directly, enabling unit testing of the parser.
• Add src/__tests__/cli.test.ts regression coverage.
• Fix the README Programmatic example, which was also wrong — it showed await diff('old.cdx.json', 'new.cdx.json') (file paths, async), but diff() takes two parsed SBOM objects and is synchronous. Updated to parse() + diff() matching the real API, and corrected the return-type comments (VersionChange[], CVEEntry[]).

Verification

$ node dist/cli.js old.json new.json            # default → works, prints text report
$ node dist/cli.js old.json new.json --format markdown   # works
$ node dist/cli.js old.json new.json --format=json       # works
$ node dist/cli.js old.json new.json --format xml        # clean error, exit 1

npm run lint, npm run typecheck, and npm test all pass (25 tests, including 5 new CLI tests).

https://claude.ai/code/session_014i1tcsdNL9uaFYgd34bTV6

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