Improve contributor onboarding: dev setup, troubleshooting, and repo coordination

[szverev]

Problem

This is a multi-repo workspace (engine + supporting packages). New contributors can struggle with:

• running the end-to-end pipeline locally
• Docker build quirks
• environment variables
• where to implement a change

Goal

Provide best-in-class contributor experience and documentation.

Scope

• Update main README with:
  • architecture overview
  • end-to-end command cycle
  • where data lives (DB tables)
• Add troubleshooting section:
  • extraction failures (expected vs actionable)
  • Discourse publish failures
  • Docker build reliability (e.g., compose bake)
• Add CONTRIBUTING.md
  • branching + commit conventions
  • how to run tests/build

Acceptance criteria

• A new contributor can run analyze → write(query) → publish in <30 minutes
• Common failure modes have clear guidance

Tasks

• README refresh
• CONTRIBUTING.md
• Troubleshooting docs