docs: tone down worktrees UI to be less aggressive

Worktrees are an advanced feature for parallel PRD work, not something
every user needs. Reframe the design to be PRD-first with worktrees
available when needed rather than front-and-center.

Key changes:
- New PRD flow only shows worktree option when another PRD is running
- Default is "use current directory", not "create worktree"
- Dashboard footer hides [E] Env and [A] All unless inside a worktree
- Picker stays as "Select PRD" (not "Select Worktree")
- Port info in header only shown when environment is running
- First-time user journey has no mention of worktrees
- Design philosophy reframed from "worktree-first" to "PRD-first"

https://claude.ai/code/session_01V27fcymZwAuZxwfqDtUxM8

Author: claude