Improve docs/index.html to better convert visitors to stars.

Changes needed

• Emphasize AI agent parallel workflow as primary use case (not just team dev)
• Quick Start step 02: update to parsec init zsh (shell integration)
• Add "Why parsec?" section showing token savings for AI agents
• Highlight competitive advantages vs worktrunk/gwq (ticket integration, PR lifecycle)
• Make setup flow smoother: install → init → start → ship
• Reference worktrunk.dev for design/messaging inspiration

Goals

• Lower perceived setup friction
• Position parsec as THE tool for AI agent workflows
• Drive GitHub stars