docs: reorganize README — lead with problem and value, not competition

Restructure README to follow: problem → value → quick start → reference.
Open with the real pain (AI agents burning tokens re-orienting in large
codebases), then what codegraph does about it, then how easy it is to
start. Move competitive comparison to a condensed table at the bottom.
Remove defensive Narsil head-to-head section. Add --format mermaid
diff-impact example from PR #58.

Author: github-actions[bot]