docs: backfill user documentation

[gregggreg]

Summary

• backfill configuration bootstrap, full command and flag coverage, providers, scheduling, service lifecycle, integrations, and troubleshooting
• distinguish built-in defaults from the generated starter configuration
• document runtime behavior accurately, including unwired project/integration schema features, provider-selection exceptions, daemon limits, and installed-service schedule limitations
• add standalone Copilot setup while clearly identifying the retired gh extension compatibility path
• keep the change documentation-only and remove unsupported guidance

Validation

• go test ./...
• go build ./cmd/nightshift
• npm run build in website (Docusaurus onBrokenLinks: throw)
• rendered-page spot checks for the corrected CLI, integration, and scheduling sections
• git diff --check
• documentation-only scope check