feat(subagent): enforce explicit summary output from subagents

[echobt]

Summary

When a subagent completes without providing a structured summary, the executor now requests an additional turn specifically asking for a summary. This ensures subagents always return properly formatted output for the orchestrator to process.

Problem

Subagents were terminating without providing a proper final summary to the orchestrator. This made it difficult for the orchestrator to:

• Know what tasks were actually completed
• Track the subagent's progress
• Coordinate with other agents

Example from user's history shows the agent only marked 2 of 6 tasks as completed and terminated without any summary output.

Solution

Following the pattern from OpenAI's Codex (--session-summary feature), we now:

1. Detect missing summaries: Added has_summary_output() function that checks for summary markers like:

   • ## Summary for Orchestrator
   • ### Tasks Completed
   • Status: COMPLETED

2. Request explicit summary: When a subagent completes without a summary, we send an additional turn with SUMMARY_REQUEST_PROMPT that explicitly asks for the structured format.

3. Combine responses: The original response is combined with the summary to provide complete output.

4. Track token usage: Token counts from both turns are properly aggregated.

Changes

• cortex-engine/src/tools/handlers/subagent/executor.rs:
  • Added SUMMARY_REQUEST_PROMPT constant
  • Added has_summary_output() function with tests
  • Modified run_subagent() to request summary turn when needed

Testing

• Added unit tests for has_summary_output()
• All existing tests pass
• Verified with cargo check and cargo fmt