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/content/docs/getting-started/installation.md

@@ -11,7 +11,7 @@ npx @colbymchenry/codegraph
 
 The installer will:
 
-- Ask which agent(s) to configure — auto-detecting installed ones from **Claude Code**, **Cursor**, **Codex CLI**, **opencode**, and **Hermes Agent**.
+- Ask which agent(s) to configure — auto-detecting installed ones from **Claude Code**, **Cursor**, **Codex CLI**, **opencode**, **Hermes Agent**, **Gemini CLI**, **Antigravity IDE**, and **Kiro**.
 - Prompt to install `codegraph` on your `PATH` (so agents can launch the MCP server).
 - Ask whether configs apply to all your projects or just this one.
 - Write each chosen agent's MCP server config plus an instructions file (e.g. `CLAUDE.md`, `.cursor/rules/codegraph.mdc`, `~/.codex/AGENTS.md`).
@@ -37,7 +37,7 @@ codegraph install --print-config codex               # print snippet, no file wr
 
 ## 2. Restart your agent
 
-Restart your agent (Claude Code / Cursor / Codex CLI / opencode / Hermes Agent) for the MCP server to load.
+Restart your agent (Claude Code / Cursor / Codex CLI / opencode / Hermes Agent / Gemini CLI / Antigravity IDE / Kiro) for the MCP server to load.
 
 ## 3. Initialize projects