Docs: add note on Codegen CLI availability in Codegen agent documentation

Included a note in the Codegen agent documentation to inform users that the Codegen CLI must be installed and available in the system PATH. This addition clarifies the prerequisites for using the agent and enhances the overall comprehensiveness of the documentation.

Author: jaruesink

.agentos/agents/:codegen-agent.md

@@ -2,7 +2,7 @@
 
 ## Purpose
 
-Take a task description or spec file, gather the relevant repository context, and produce a **well-structured prompt** for Codegen.  
+Take a task description or spec file, gather the relevant repository context, and produce a **well-structured prompt** for Codegen.
 This agent does not execute any implementation work — its sole role is to build comprehensive context and craft the optimal prompt for the Codegen CLI.
 
 The agent then provides the structured prompt to be executed via:
@@ -11,6 +11,8 @@ The agent then provides the structured prompt to be executed via:
 codegen agent --prompt "<full-structured-prompt>"
 ```
 
+**Note:** You can assume the `codegen` CLI is installed and available in the system PATH.
+
 ## Workflow
 
 **Important:** This workflow focuses entirely on context building and prompt construction. The agent performs no code implementation, file modifications, or actual development work.
@@ -64,6 +66,7 @@ codegen agent --prompt "<full-structured-prompt>"
 - If required context (e.g., constitution, tech-stack, spec) is missing, mark with `[NEEDS CLARIFICATION]`
 - If file relevance is uncertain, output your file map and ask for confirmation
 - Do not provide the prompt to Codegen until all sections are complete and well-structured
+- If Codegen CLI is not available, the agent should halt and inform the user to install it before proceeding
 
 ---