bug(start): preserve argv boundaries instead of re-shelling joined command

[pinion05]

Summary

agentty start currently collapses the original argv into a single space-joined string and the worker runs it via shell -lc.

Why this is a problem

• Quoted arguments lose their original boundaries.
• Empty arguments cannot be represented correctly.
• Commands containing spaces or literal quotes can be reinterpreted by the shell.

Impact

Common invocations like agentty start python3 -c "print("a b")" can behave differently from the user's original intent.

Suggested direction

• Keep the command as file + args[] through the CLI/runtime boundary.
• Spawn the target command directly instead of re-parsing a reconstructed shell string.
• Add regression tests for quoted args, empty args, and literal quotes.

Relevant code

• src/index.ts#parseStartOptions
• src/worker.ts#spawn(shell, ["-lc", spec.command], ...)