Initialize public project guides and harden migrations

[mayvqt]

Summary

• turn the root README into a concise documentation index
• add public capability, architecture, data, validation, operations, and documentation guides
• apply every outstanding supported SQLite migration during upgrades
• avoid duplicate CI runs on main while retaining the release gate

Validation

• go test ./internal/db
• Markdown link targets and git diff --check
• full GitHub Actions CI required before merge