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>