Restructure README documentation

[pasunboneleve]

Summary

• Lead the README with CLI evaluation evidence instead of the stencil image
• Move the stencil metaphor into a later name section
• Clarify per-skill eval discovery and update CLI help wording
• Add light and dark CLI output SVG previews

Validation

• git diff --check
• rsvg-convert docs/images/skilpel-output-dark.svg -o /tmp/skilpel-output-dark.png
• rsvg-convert docs/images/skilpel-output-light.svg -o /tmp/skilpel-output-light.png
• GOCACHE=/home/dmvianna/src/projects/skilpel/.gocache CCACHE_DIR=/home/dmvianna/src/projects/skilpel/.ccache go test ./internal/skilpel