Add Node version guard for unsupported runtimes

[iamtoruk]

Summary

• Split CLI into a 15-line launcher (src/cli.ts) + full CLI (src/main.ts)
• Launcher checks for Node >= 22.13.0 before dynamically importing main bundle
• Users on Node 18 get a clear upgrade message instead of a cryptic SyntaxError: Invalid regular expression flags from string-width
• Aligned engines field to >=22.13.0 to match the actual floor
• Cross-platform build (no cp dependency)

Closes #232.

Why two files?

The crash is a parse error -- Node 18 can't even load the bundle because string-width uses the /v regex flag (Node 20+). A runtime check in the same file won't help because the file fails to parse before any code runs. The launcher must be a separate file that Node 18 can parse.

Build output

• dist/cli.js -- 400-byte launcher (version check + dynamic import)
• dist/main.js -- 667KB bundle (the actual CLI)

Test plan

• 678/678 tests pass
• node dist/cli.js --version works
• node dist/cli.js report --format json produces valid output
• Version gate correctly blocks 18.x, 22.0-22.12
• Version gate correctly allows 22.13+, 23.x, 24.x
• Build works cross-platform (no cp dependency)
• Dynamic import has .catch() for proper error handling