feat: add --block flag to poc-agent read

The --block flag makes poc-agent read block until a complete response
is received (detected by a new user input line starting with '>'),
then exit. This enables smoother three-way conversations where one
instance can wait for the other's complete response without polling.

The implementation:
- Added cmd_read_inner() with block parameter
- Modified socket streaming to detect '>' lines as response boundaries
- Added --block CLI flag to Read subcommand

The --follow flag continues to stream indefinitely.
The --block flag reads one complete response and exits.
Neither flag exits immediately if there's no new output.

poc-agent/src/cli.rs

@@ -62,6 +62,9 @@ pub enum SubCmd {
         /// Stream output continuously instead of exiting
         #[arg(short, long)]
         follow: bool,
+        /// Block until a complete response is received, then exit
+        #[arg(long)]
+        block: bool,
     },
     /// Send a message to the running agent
     Write {