fix: improve error messaging for agent output validation failures

[Mr-Neutr0n]

Summary

Improve error messages when agents fail to create required deliverable files, making it easier to diagnose and fix common issues.

Problem

When the pre-recon agent (or other agents) fail to create required deliverable files, users see a generic error:

Agent pre-recon failed output validation after 3 attempts

This doesn't help users understand what went wrong or how to fix it.

Solution

Add troubleshooting tips and more detailed logging:

1. Troubleshooting tips in error message (for pre-recon agent):

   • Check if repository path is accessible
   • If using ROUTER mode, ensure model supports tool use
   • Check agent logs location
   • Try Claude models (recommended)
   • Ensure target URL is reachable from Docker

2. Log expected file path when validation fails:

   Expected file: /path/to/repo/deliverables/code_analysis_deliverable.md
   

Common causes this helps diagnose

• Using router mode with models that don't follow instructions well
• Repository path not accessible from Docker container
• Target URL not reachable (should use host.docker.internal for local apps)

Test plan

• Run Shannon with an intentionally inaccessible repo path
• Verify improved error message appears with troubleshooting tips
• Verify expected file path is logged on validation failure

Fixes #62