docs: add .repo-docs directory

[vexa-tski]

Summary

• Adds comprehensive .repo-docs/ directory with standardized documentation for the data-gen ETL pipeline
• Covers project overview, architecture (pipeline stages, component map, data flow), development setup, integration map (14+ downstream repos), conventions, troubleshooting, and 4 architectural decision records
• Generated from codebase analysis -- all commands, paths, and descriptions verified against actual repo contents

Files Added

File	Description
index.md	Overview, quick reference, repo structure, reading guide
project.md	Domain context, glossary, feature inventory, constraints
architecture.md	Pipeline stages, component map, data flow, dependencies
development.md	Prerequisites, setup, common workflows, debugging
integration.md	Upstream/downstream dependency map, data contracts
conventions.md	Naming patterns, code style, commit conventions
troubleshooting.md	Do-NOTs, known issues, common errors, debug playbooks
decisions/README.md	Decision log index and ADR template
decisions/*.md	4 ADRs: HTML round-trip, Just orchestrator, SCC classification, multiple output variants

Test plan

• Review generated docs for accuracy against current codebase
• Verify all referenced commands and file paths exist
• Confirm no sensitive information is included

🤖 Generated with Claude Code