feat(installer): add Gemini CLI + Antigravity IDE targets (#399) (#458)

`codegraph install` now detects and configures two more agents:

- Gemini CLI / Antigravity CLI — `~/.gemini/settings.json` (or
  `./.gemini/settings.json`) + `~/.gemini/GEMINI.md` (or project-root
  `./GEMINI.md`). Preserves pre-existing top-level settings like
  `security.auth` and sibling MCP servers.

- Antigravity IDE — writes to Antigravity's unified MCP config at
  `~/.gemini/config/mcp_config.json` (post-migration, detected via
  the `.migrated` marker Antigravity drops). Falls back to the
  legacy `~/.gemini/antigravity/mcp_config.json` on pre-migration
  builds; install migrates a stale legacy entry, uninstall sweeps
  both. Antigravity-managed sibling fields (e.g. the `disabled` flag
  added when users disable a server through the UI) survive re-install.

  Two Antigravity-specific quirks the target handles:
  1. Entries with `type: "stdio"` are silently rejected by
     Antigravity's MCP scanner; we omit the field for this target.
  2. macOS GUI apps launched from Dock/Finder get a stripped PATH
     that excludes nvm — a bare `codegraph` command name fails to
     spawn even when `which codegraph` works in the user's shell.
     The target resolves `codegraph` to its absolute path at install
     time on macOS. Linux + Windows are unaffected.

End-to-end validated:
- macOS: real Gemini CLI v0.43 via tmux — `/mcp` shows codegraph with
  all 10 tools, `codegraph_status` executes and returns real index
  state. Real Antigravity IDE shows codegraph under Customizations
  after restart.
- Linux (Docker node:22-bookworm) + Windows (Parallels Win11): 116
  installer tests pass; CLI install + uninstall round-trip verified.

Test coverage: the new targets inherit the existing parameterized
contract (idempotent install, sibling preservation, install/uninstall
round-trip). Plus 14 target-specific tests covering migration-marker
detection, legacy→unified entry migration, `disabled` flag
preservation, the `type` field omission, gemini+antigravity
coexistence in the same `~/.gemini/`, and macOS-only path resolution.
Full suite: 972 passing.

Closes #399.

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

README.md

@@ -2,7 +2,7 @@
 
 # CodeGraph
 
-### Supercharge Claude Code, Cursor, Codex, OpenCode, and Hermes Agent with Semantic Code Intelligence
+### Supercharge Claude Code, Cursor, Codex, OpenCode, Hermes Agent, Gemini CLI, and Antigravity IDE with Semantic Code Intelligence
 
 **~35% cheaper · ~70% fewer tool calls · 100% local**
 
@@ -21,6 +21,8 @@
 [![Codex CLI](https://img.shields.io/badge/Codex_CLI-supported-blueviolet.svg)](#supported-agents)
 [![opencode](https://img.shields.io/badge/opencode-supported-blueviolet.svg)](#supported-agents)
 [![Hermes Agent](https://img.shields.io/badge/Hermes_Agent-supported-blueviolet.svg)](#supported-agents)
+[![Gemini CLI](https://img.shields.io/badge/Gemini_CLI-supported-blueviolet.svg)](#supported-agents)
+[![Antigravity IDE](https://img.shields.io/badge/Antigravity_IDE-supported-blueviolet.svg)](#supported-agents)
 
 </div>
 
@@ -43,7 +45,7 @@ npx @colbymchenry/codegraph        # zero-install, or:
 npm i -g @colbymchenry/codegraph
 ```
 
-<sub>CodeGraph bundles its own runtime — nothing to compile, no native build, works the same everywhere. The interactive installer auto-configures your agent(s) — Claude Code, Cursor, Codex CLI, opencode, Hermes Agent.</sub>
+<sub>CodeGraph bundles its own runtime — nothing to compile, no native build, works the same everywhere. The interactive installer auto-configures your agent(s) — Claude Code, Cursor, Codex CLI, opencode, Hermes Agent, Gemini CLI, Antigravity IDE.</sub>
 
 ### Initialize Projects
 
@@ -230,10 +232,10 @@ npx @colbymchenry/codegraph
 ```
 
 The installer will:
-- Ask which agent(s) to configure — auto-detects installed ones from: **Claude Code**, **Cursor**, **Codex CLI**, **opencode**, **Hermes Agent**
+- Ask which agent(s) to configure — auto-detects installed ones from: **Claude Code**, **Cursor**, **Codex CLI**, **opencode**, **Hermes Agent**, **Gemini CLI**, **Antigravity IDE**
 - 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 + an instructions file (e.g. `CLAUDE.md`, `.cursor/rules/codegraph.mdc`, `~/.codex/AGENTS.md`)
+- Write each chosen agent's MCP server config + an instructions file (e.g. `CLAUDE.md`, `.cursor/rules/codegraph.mdc`, `~/.codex/AGENTS.md`, `~/.gemini/GEMINI.md`)
 - Set up auto-allow permissions when Claude Code is one of the targets
 - Initialize your current project (local installs only)
 
@@ -256,7 +258,7 @@ 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) for the MCP server to load.
 
 ### 3. Initialize Projects