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 Pydantic and 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 and commit conventions agreed in past reviews.
• Fixes drift: the integration vs. E2E test description and the architecture notes.
• The test READMEs cover common setup failures and platform timing quirks.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.05%. Comparing base (3ed1a48) to head (389d37f).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1180   +/-   ##
=======================================
  Coverage   93.05%   93.05%           
=======================================
  Files          54       54           
  Lines        3671     3671           
=======================================
  Hits         3416     3416           
  Misses        255      255           

Flag	Coverage Δ
e2e	31.73% <ø> (ø)
integration	51.70% <ø> (ø)
unit	89.78% <ø> (ø)

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[apify-approval-mirror]

Mirroring approval from @Mantisus.