docs: add Kiro to README + site docs (and fill in Gemini/Antigravity gaps) (#474)

PR #473 added the Kiro installer target but missed updating the README
and site documentation. This sweeps both:

- README: hero H3, badges row, installer subtitle, auto-detect list,
  restart line, Supported Agents bullet list, footer tagline.
- site/: integrations.md supported-agents list, installation.md
  auto-detect + restart lines, quickstart.md installer subtitle,
  introduction.md tagline, guides/indexing.md auto-sync paragraph,
  pages/index.astro MCP feature card.

The Supported Agents section in README and integrations.md were also
already stale from PR #399 — they didn't list Gemini CLI or Antigravity
IDE. Fixed those alongside.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

site/src/pages/index.astro

@@ -93,8 +93,8 @@ const install = 'npx @colbymchenry/codegraph';
 						</svg>
 						<h2>MCP server</h2>
 						<p>
-							Expose the graph to Claude Code, Cursor, Codex, opencode, and Hermes
-							over MCP — agents answer in a handful of calls.
+							Expose the graph to Claude Code, Cursor, Codex, opencode, Hermes,
+							Gemini, Antigravity, and Kiro over MCP — agents answer in a handful of calls.
 						</p>
 					</article>
 					<article class="feature">