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 0% with 27 lines in your changes missing coverage. Please review.

Project coverage is 51.57%. Comparing base (3b580de) to head (3151109).

Files with missing lines	Patch %	Lines
lumen/ai/agents.py	0.00%	25 Missing ⚠️
lumen/ai/controls.py	0.00%	1 Missing ⚠️
lumen/ai/coordinator.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #882      +/-   ##
==========================================
- Coverage   51.66%   51.57%   -0.10%     
==========================================
  Files         108      108              
  Lines       13697    13721      +24     
==========================================
  Hits         7076     7076              
- Misses       6621     6645      +24     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.