Implement per-step validation and replanning

[philippjfr]

Description

• Add a post-step validation hook in the plan execution loop (after each primary actor task output is available).
• If step validation fails/incomplete, gather compact replanning context (original user goal, completed steps, failed/current step, validation findings, available agents/tools, current execution context).
• Invoke Planner in partial replan mode using a new prompt template and update only the remaining plan segment.
• Continue execution from the newly spliced plan tail.

AI Disclosure

• This PR contains AI-generated content.
  • I have tested all AI-generated content in my PR.
  • I take responsibility for all AI-generated content in my PR.

Wrote detailed specification of what I wanted the validation / replan loop to look like and reviewed the generated code in detail.

Tools and Models: 5.3 Codex

Checklist

• Tests added and are passing
• Added documentation

[codecov]

Codecov Report

❌ Patch coverage is 90.70796% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.95%. Comparing base (43c9c5d) to head (e299589).
⚠️ Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
lumen/ai/coordinator/base.py	82.17%	18 Missing ⚠️
lumen/tests/ai/test_coordinator.py	97.16%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1769      +/-   ##
==========================================
+ Coverage   68.75%   68.95%   +0.19%     
==========================================
  Files         171      171              
  Lines       28979    29189     +210     
==========================================
+ Hits        19924    20126     +202     
- Misses       9055     9063       +8     

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