docs: backfill user documentation

[gregggreg]

Summary

• backfill configuration bootstrap, complete command and flag coverage, scheduling, service lifecycle, providers, integrations, and troubleshooting
• distinguish built-in defaults from generated starter files and document configuration precedence and merge behavior
• document actual runtime boundaries for custom tasks, project patterns, integration readers, daemon limits, and installed-service schedule translation
• update Copilot setup for standalone Copilot and the current built-in gh copilot command while identifying Nightshift's stale direct-task availability gate
• add Troubleshooting to the Docusaurus navigation and keep the change documentation-only

Validation

• go test ./...
• go build ./cmd/nightshift
• npm run build in website with onBrokenLinks: 'throw'
• rendered-page checks for navigation and corrected CLI/provider guidance
• source-to-doc command, flag, config-key, and task-slug coverage checks
• git diff --check
• documentation-only scope check

Supersedes the documentation content in draft #198 after a fresh implementation audit.