docs(architecture): add repository guides

[ekalinin]

Summary

This PR adds two documentation files:

• AGENTS.md - repository-specific contributor guidance for project structure, development commands, coding style, testing, commits, pull requests, security, and documentation maintenance.
• ARCHITECTURE.md - an overview of the Clean/hexagonal architecture, package hierarchy, component relationships, dependencies, configuration, and document-processing flows.

No production code is changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.16%. Comparing base (112af89) to head (eeb9088).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #67   +/-   ##
=======================================
  Coverage   82.16%   82.16%           
=======================================
  Files          27       27           
  Lines         673      673           
=======================================
  Hits          553      553           
  Misses         77       77           
  Partials       43       43           

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