Update README and add template placeholder comments

[threeal]

Summary

Rewrites README.md with a cleaner structure and more useful content for template users:

• Replaces the generic intro with a focused description of what the template provides
• Splits the old numbered "Getting Started" into a top-level quick-start blurb ("Getting Started") and a dedicated "Setup" section with explicit shell commands for pnpm install and lefthook install
• Rewrites "Customizing" to use bold headings per file/folder with more precise descriptions, and drops config files that users rarely need to touch
• Adds a "Development" section explaining how to use the pre-commit hook day-to-day
• Adds a "Testing" section with the pnpm vitest run command and a note about 100% coverage enforcement
• Adds a "CI" section describing what the workflow does
• Adds a "Releasing" section covering version bumping, tagging, and pnpm publish
• Adds a "Framework-Specific Templates" section linking to related starters (Action Starter, Discord Bot Starter)