docs: consolidate contributing guide and agent rules

[vdusek]

Makes CONTRIBUTING.md the single source of development conventions. .rules.md (the target of CLAUDE.md, AGENTS.md and GEMINI.md) links and imports it and keeps only the architecture notes.

• Removes duplication between .rules.md, CONTRIBUTING.md, README.md and pyproject.toml. Poe tasks have help texts, so uv run poe replaces the task table.
• Adds code style, test, docs and commit conventions agreed in past reviews, plus a section on generated models.
• Fixes drift in the architecture notes.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.46%. Comparing base (ef8f088) to head (9a041a5).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1124   +/-   ##
=======================================
  Coverage   95.46%   95.46%           
=======================================
  Files          61       61           
  Lines        5935     5935           
=======================================
  Hits         5666     5666           
  Misses        269      269           

Flag	Coverage Δ
integration	90.81% <ø> (+0.21%)	⬆️
unit	88.87% <ø> (ø)

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[apify-approval-mirror]

Mirroring approval from @Mantisus.