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

[colbymchenry]

Summary

Adds two installer targets so codegraph install configures Gemini CLI and the Antigravity IDE out of the box, closes #399.

• Gemini CLI (covers the rebranded Antigravity CLI too — same config file): ~/.gemini/settings.json + ~/.gemini/GEMINI.md (or project-local equivalents). 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 itself drops). Falls back to the legacy ~/.gemini/antigravity/mcp_config.json for pre-migration builds; install migrates a stale legacy entry, uninstall sweeps both. Antigravity-managed sibling fields (e.g. the "disabled": true flag added when a user disables a server through the IDE) survive re-install.

Antigravity quirks the target handles

These caused codegraph to silently fail to appear in Antigravity's MCP server list on initial implementation; both are now in the installer.

1. type: "stdio" is rejected — Antigravity silently drops MCP entries that carry this field. The working entries it manages itself omit it, so the antigravity target omits it too. (All other targets keep type: "stdio" — only Antigravity is picky about this.)
2. macOS GUI app PATH problem — macOS gives Dock/Finder-launched apps a stripped PATH (/usr/bin:/bin:/usr/sbin:/sbin) that doesn't include 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 GUI apps inherit user PATH and Windows reads env PATH directly, so both keep the bare command.

End-to-end validation

• macOS — real Gemini CLI v0.43.0 via tmux: /mcp reports 🟢 codegraph - Ready (10 tools); codegraph_status executed and returned the real index state (188 files, 2854 nodes). Real Antigravity IDE shows codegraph in Customizations → Installed MCP Servers after restart.
• Linux (Docker node:22-bookworm) — 116 installer tests pass; CLI install + uninstall round-trip verified, including the unified-path detection.
• Windows (Parallels Win11) — 116 installer tests pass; CLI install + uninstall round-trip verified.

Test plan

• Parameterized contract tests pick up both new targets (idempotent install, sibling preservation, install/uninstall round-trip)
• Gemini: preserves security.auth, GEMINI.md sibling content; local path writes to ./.gemini/settings.json + project-root ./GEMINI.md
• Antigravity: writes to legacy path without marker, writes to unified path WITH marker, writes to unified path when the unified file already exists (even without marker), entry has no type field, sibling disabled flag preserved, install migrates a legacy entry to unified, uninstall sweeps both paths
• Cross-target: gemini + antigravity coexist in ~/.gemini/; uninstalling one preserves the other's MCP entry
• Full suite: 972 passing on macOS / Linux / Windows

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