docs(readme): show the Trail-code banner

[shhdwi]

The repo is now Trail-Code, so the README's top image should say so before #558 merges.

• Replaces the graft hero with the Trail-code banner from the README rewrite: light and dark versions, picked by the viewer's GitHub theme.
• Nothing else in the README changes. The tagline, badges, benchmarks and install steps still describe graft as it ships today, so nothing points at trail commands before they exist on main.

🤖 Generated with Claude Code

[trailhq-graft]

🌱 graft blast radius

Nothing outside this diff depends on it. 0 areas changed; no indexed dependents at depth 2.

⚠️ 3 changed files not in the graph (README.md, assets/trail-code-banner-dark.png, assets/trail-code-banner.png) — no parser claims the extension, or the index predates the file.

_{graft blast · origin/main...HEAD · depth 2 · 3 changed files}

Open the interactive graph → — click an area to see its dependent symbols at file:line.