Update README with build agent prompt instructions

Added note about prompting the build agent if experimental plan mode is not enabled.

Author: timrichardson

README.md

@@ -3,7 +3,7 @@
 `opencode-planner` is an OpenCode plugin that adds a dedicated `plan` agent for read-only planning before implementation. It's based on the experimental plan agent. That is, it likes to use sub-agents and a structured approach to planning.
 It asks clarifying questions, and produces a markdown file. When Plannotator is installed, it can submit the finished plan for richer review. Without Plannotator, it falls back to a normal chat-based review handoff.
 
-After review, the agent can hand back to implementation mode by calling `plan_exit` only when the host runtime exposes that tool. In current OpenCode builds, that means experimental plan mode must be enabled and the client must be `cli`.
+After review, the agent can hand back to implementation mode by calling `plan_exit` only when the host runtime exposes that tool. In current OpenCode builds, that means experimental plan mode must be enabled and the client must be `cli`. If it's not enabled, you need to prompt the build agent to start work.
 
 Repository: <https://github.com/timrichardson/opencode-planner>