Add worktree guide for onboarding (#7)

A tutorial covering what git worktrees are, why they matter for running
many Claude Code agents in parallel, and how Harness turns them into a
usable workflow. Aimed at developers new to the multi-agent flow.

Lives in-app as a new Guide view (accessible from the repo-select
screen and Settings > Help) and on the marketing site at
docs/guide.html. Includes four programmatic diagrams — worktree
anatomy, collision vs isolation, a Harness sidebar mockup, and a
parallel workflow timeline — so there's no reliance on raster assets.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: frenchie4111