Link GitHub repository with an icon (fleet policy #91)

[sarthakagrawal927]

What changed

The public proof site (web/src/pages/index.astro, served at https://mashup.highsignal.app) had a header nav link with visible text GitHub ↗. Replaced it in place with an icon-only link (inline GitHub-mark SVG, aria-label/title="GitHub repository", target="_blank" rel="noopener noreferrer"), per fleet policy sass-maker/saas-maker#91 ("No GitHub text, please").

Left untouched, deliberately:

• The footer's "Local editor guide ↗" link and the release-boundary section's "Run Mashup locally" / "Read the release record" links all resolve to github.com/sass-maker/mashup sub-paths, but each carries distinct, meaningful copy describing a specific action (not a generic "GitHub"/"Source" badge), so they're out of scope for this policy.
• /editor and /visual-lab are robots.txt: noindex local-operator tool pages with no header/footer/site chrome at all (per their own copy: "Local operator tool"); adding a GitHub badge there would mean building new chrome, which is out of scope for this change.

Verification

• cd web && pnpm install && pnpm check (astro check) — pass, 0 errors/warnings/hints
• pnpm build (astro build) — pass
• Confirmed the built web/dist/index.html contains the icon link, no leftover generic "GitHub" text, and the three distinct CTA links are intact.
• Only web/ (Astro) changed; the Python side (uv sync/pytest/ruff) is untouched by this diff.

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