sms_tool: support asynchronous AT replies

[dormancygrace]

What changed

• add -w <milliseconds> for the at command
• after the terminal OK, keep collecting asynchronous modem output until the configured quiet period expires
• keep the current zero-delay behavior by default
• document the AT+QTEMP use case

Why

Some Quectel modems acknowledge vendor commands such as AT+QTEMP with
OK before emitting their actual result. sms_tool exited immediately at
that first OK, so the useful response was lost. The delay is explicit
because the modem gives no generic indication of when asynchronous output is
complete.

This completes the unresolved behavior described in #11.

Validation

• native release build
• MIPS 24Kc/musl OpenWrt cross-compilation
• pseudo-terminal integration test: data emitted 150 ms after OK is absent
  without -w and captured with -w 300
• invalid and out-of-range delay validation
• git diff --check