fix(mcp): prevent stdout pollution in MCP stdio protocol

Root Cause:
Lines 1133-1135 (dual transport) and 1140-1142 (daemon mode) wrote
directly to stdout without checking if stdout is attached to a TTY.
This violated the MCP protocol specification which requires stdout
to contain ONLY JSON-RPC messages when in stdio mode.

Impact:
- MCP clients couldn't connect: "Failed to reconnect to codegraph"
- JSON-RPC parsing failed due to text preceding JSON messages
- Protocol violation prevented any MCP communication

Detection:
Dual transport: println!("✓ MCP server started with dual transport");
Daemon mode:    println!("Running in daemon mode");

Fix:
Wrapped all println! calls in handle_start() with atty::is(Stream::Stdout)
checks, matching the pattern used elsewhere in the codebase (handle_stop,
etc.). Messages now only appear when stdout is a terminal, not when it's
being used for MCP JSON-RPC communication.

Verified:
- Build successful
- Follows existing pattern from lines 1154-1155 (handle_stop)
- Only affects dual transport and daemon mode startup messages
- Other println! calls are in CLI-only commands (status, config, index)

Author: jruokola

crates/codegraph-mcp/src/bin/codegraph.rs

@@ -1130,16 +1130,20 @@ async fn handle_start(
                     pid_file.clone(),
                 )
                 .await?;
-            println!("✓ MCP server started with dual transport");
-            println!("  STDIO: buffer size {} (PID: {})", buffer_size, stdio_pid);
-            println!("  HTTP: http://{}:{} (PID: {})", host, port, http_pid);
+            if atty::is(Stream::Stdout) {
+                println!("✓ MCP server started with dual transport");
+                println!("  STDIO: buffer size {} (PID: {})", buffer_size, stdio_pid);
+                println!("  HTTP: http://{}:{} (PID: {})", host, port, http_pid);
+            }
         }
     }
 
     if daemon {
-        println!("Running in daemon mode");
-        if let Some(ref pid_file) = pid_file {
-            println!("PID file: {:?}", pid_file);
+        if atty::is(Stream::Stdout) {
+            println!("Running in daemon mode");
+            if let Some(ref pid_file) = pid_file {
+                println!("PID file: {:?}", pid_file);
+            }
         }
     }