docs: backfill missing CLI documentation

[marcus]

Summary

• align the Docusaurus command reference with the current Cobra help surface, including notes, sync, diagnostics, sessions, comments, deleted, resume, and handoff commands
• add focused user docs for notes, sync/collaboration, and system diagnostics, and wire them into the sidebar
• fix stale README references for the td image, SPEC link, local-first wording, and new docs pages

Accuracy notes

• sync project roles are documented as the implemented values: owner, writer, reader
• diagnostics documents the implemented td doctor command only; no unsupported doctor subcommands or JSON flags are described
• delegated review-only flags from other branches are not documented because they are not in this branch's CLI help

Validation

• npm run build in website
• go test ./cmd ./internal/features ./internal/config
• commit hook: go vet and go build