docs: update AGENTS workflows for s3 testing

[shaypal5]

Summary

• update AGENTS.md backend references to include S3 where workflows and markers were outdated
• add missing requirements_s3.txt references in command lists
• document ./scripts/test-local.sh s3 and -f/--files option from local test workflow
• correct the external-backend pytest example to also exclude redis

Scope

• docs-only change (AGENTS.md)
• no runtime code changes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (acf18b5) to head (dc029cc).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #363   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        12           
  Lines         1701      1701           
  Branches       213       213           
=========================================
  Hits          1701      1701           

Flag	Coverage Δ
local	58.55% <ø> (ø)
mongodb	41.03% <ø> (+0.11%)	⬆️
postgres	43.85% <ø> (ø)
redis	47.14% <ø> (ø)
s3	42.85% <ø> (ø)

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