Add guard against concurrent prompt() calls

Agent.prompt() and Agent.continue() now throw if called while already
streaming, preventing race conditions and corrupted state. Use
queueMessage() to queue messages during streaming, or await the
previous call.

AgentSession.prompt() has the same guard with a message directing
users to queueMessage().

Ref #403

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- `AgentSession.prompt()` now throws if called while the agent is already streaming, preventing race conditions. Use `queueMessage()` to queue messages during streaming.
+
 ## [0.31.1] - 2026-01-02
 
 ### Fixed