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 {

poc-agent/src/main.rs

@@ -83,8 +83,8 @@ async fn main() {
 
     // Subcommands that don't launch the TUI
     match &cli.command {
-        Some(cli::SubCmd::Read { follow }) => {
-            if let Err(e) = observe::cmd_read(*follow, cli.debug).await {
+        Some(cli::SubCmd::Read { follow, block }) => {
+            if let Err(e) = observe::cmd_read_inner(*follow, *block, cli.debug).await {
                 eprintln!("{:#}", e);
                 std::process::exit(1);
             }

poc-agent/src/observe.rs

@@ -69,6 +69,11 @@ fn cursor_path() -> PathBuf { session_dir().join("read-cursor") }
 
 /// Print new output since last read. With -f, also stream live from socket.
 pub async fn cmd_read(follow: bool, debug: bool) -> anyhow::Result<()> {
+    cmd_read_inner(follow, false, debug).await
+}
+
+/// Print new output since last read. With -f, stream live. With block, wait for one response.
+pub async fn cmd_read_inner(follow: bool, block: bool, debug: bool) -> anyhow::Result<()> {
     use std::io::{Read, Seek, SeekFrom, Write};
 
     let log = log_path();
@@ -91,20 +96,20 @@ pub async fn cmd_read(follow: bool, debug: bool) -> anyhow::Result<()> {
             f.read_to_string(&mut buf)?;
             print!("{}", buf);
             let _ = std::io::stdout().flush();
-        } else if !follow {
+        } else if !follow && !block {
             println!("(nothing new)");
         }
         let _ = std::fs::write(&cursor, len.to_string());
-    } else if !follow {
+    } else if !follow && !block {
         println!("(no log yet — is poc-agent running?)");
         return Ok(());
     }
 
-    if !follow {
+    if !follow && !block {
         return Ok(());
     }
 
-    // -f: connect to socket for live output
+    // -f or --block: connect to socket for live output
     let sock = socket_path();
     let stream = UnixStream::connect(&sock).await
         .map_err(|e| anyhow::anyhow!(
@@ -122,6 +127,11 @@ pub async fn cmd_read(follow: bool, debug: bool) -> anyhow::Result<()> {
             Ok(_) => {
                 print!("{}", line);
                 let _ = std::io::stdout().lock().flush();
+                
+                // In blocking mode, stop when we see a new user input (line starting with ">")
+                if block && line.trim_start().starts_with('>') {
+                    break;
+                }
             }
             Err(_) => break,
         }