docs: complete self-hosting guide and project documentation

[mariuspruvot]

Summary

• README.md: full rewrite -- architecture diagram, how-it-works flow, skills table, documentation index. Replaces the bare-bones original.
• docs/self-hosting.md: step-by-step guide from zero to running instance -- GitHub App creation, env config, 3 deployment options (Docker Compose, Coolify, any VPS), Claude credential setup, testing, and troubleshooting.
• docs/architecture.md: mermaid system diagram, request flows (webhook, session start, multi-turn), module map with all endpoints, container lifecycle state machine, stream-json protocol, FIFO mechanism, security model, and database schema overview.
• docs/creating-skills.md: walkthrough of building a custom skill from scratch (security-audit example), template placeholders, config reference, local testing, output conventions.
• CONTRIBUTING.md: dev setup, code style, testing conventions, PR process, module layout pointer.
• .env.example: improved comments with generation commands for every secret, grouped sections, fixed DATABASE_URL default (was localhost, now db for docker-compose).
• CLAUDE.md: synced with main (mypy, CI coverage), added pointer to human-facing docs.

Test plan

• All internal markdown links verified (20/20 resolve)
• make lint passes (ruff + eslint)
• Follow self-hosting guide steps mentally -- every command is copy-pasteable
• Verify README renders correctly on GitHub

[helprs-local-mariuspruvot]

helPRs session created for this PR.

Skill: challenge-me | Open session