Sharpen README product front door

[codeforester]

Summary

• Rework the top of the README around Base's developer product value: workspace control plane, first-run commands, common project loop, and success signals.
• Add a concise "How Base Fits" section that explains how Base relates to Homebrew, mise, and project-owned logic.
• Rename the lower detailed quick-start section to Installation Details so the first screen stays focused while deeper setup details remain available.

Validation

sed -n '1,110p' README.md
rg -n "^## Quick Start|^## Installation Details|^## Start Here|^## How Base Fits" README.md
git diff --check

Closes #260