[codex] Add terminal graph landing page

[eric-tramel]

What changed

• Replaces the plain docs introduction with a dark Terminal Graph landing page for Moraine.
• Adds a responsive trace-map hero showing agent harnesses flowing into Moraine and out to ClickHouse, Monitor UI, and MCP Search.
• Adds concise sections for why Moraine exists, how to start locally, and supported harnesses.
• Adds checked-in screenshots for review context and PR rendering.

Why

The GitHub Pages entry point now gives new visitors a clearer visual explanation of what Moraine does and how to use it without overloading them before they reach the deeper docs.

Screenshots

Desktop

Mobile

Interactions checked

• Opened the landing page at desktop viewport and captured the hero screenshot.
• Opened the landing page at mobile viewport and captured the responsive stacked layout screenshot.
• Clicked the hero Quickstart CTA and verified navigation to /quickstart.html.
• Clicked the hero MCP Search CTA and verified navigation to /agent-mcp-search/index.html.
• Verified no page-level horizontal overflow at the checked desktop and mobile viewport sizes.

Operational impact

Docs-only change. No runtime, config, migration, or service behavior changes.

Validation

• make docs-build -> passed, no issues found.
• git diff --check -> passed.
• Browser preview at desktop and mobile widths for layout, CTA navigation, screenshot capture, and overflow checks.