feat: add browser-safe /protocol subpath export (#11)

Adds a ./protocol entry to the package.json exports map, pointing at
the existing src/protocol.ts module (built to dist/protocol.js by the
existing tsc build). This module exports PacketReader, MessageType,
Packet, and all encode/decode helpers with zero Node built-in deps
beyond Buffer — making it safe for browser bundlers.

Motivation: consumers like forge maintain a copy of the wire protocol
for browser-side xterm.js packet parsing. With this subpath they can
import from @myobie/pty/protocol instead, keeping the protocol in one
place.