Add docs list agent

[ahuang11]

Shows users a list of documents they uploaded.

I think provides should accept sources|document_sources. Right now, I work around that by doing:

if p not in self._memory]
            if unprovided and not any(unprovided):
                step.failed_title = f"{agent_name} did not provide {', '.join(unprovided)}. Aborting the plan."

Since there's an RC out, maybe merge this after release.

[codecov]

Codecov Report

Attention: Patch coverage is 42.30769% with 15 lines in your changes missing coverage. Please review.

Project coverage is 57.98%. Comparing base (861d49f) to head (c32f52d).
Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
lumen/ai/agents.py	39.13%	14 Missing ⚠️
lumen/ai/controls.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #882      +/-   ##
==========================================
- Coverage   58.40%   57.98%   -0.42%     
==========================================
  Files         109      109              
  Lines       13915    14057     +142     
==========================================
+ Hits         8127     8151      +24     
- Misses       5788     5906     +118     

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[ahuang11]