fix(installer): resolve Antigravity codegraph command out of fnm's ephemeral per-shell dir

[kevocodes]

Overview

On macOS with fnm, the Antigravity installer writes an MCP command that points into an ephemeral, per-shell directory. It works at install time but dangles once the installing shell exits, so Antigravity can no longer spawn the codegraph server.

Fixes #1443.

Root cause

resolveCodegraphCommand() (src/installer/targets/antigravity.ts) resolves the absolute path via command -v codegraph. Under fnm, an interactive shell resolves codegraph to ~/.local/state/fnm_multishells/<pid>_<ts>/bin/codegraph — a symlink directory fnm creates per shell and removes when that shell exits. That path is written verbatim into ~/.gemini/config/mcp_config.json.

Change

Canonicalize the resolved path's containing directory with fs.realpathSync, then re-join the basename:

return path.join(fs.realpathSync(path.dirname(resolved)), path.basename(resolved));

• Escapes the ephemeral fnm_multishells/<pid> dir, so the written path outlives the installing shell. For fnm it yields ~/.local/share/fnm/node-versions/<ver>/installation/bin/codegraph.
• Preserves the codegraph basename. Some agents validate the MCP command by filepath.Base, so keeping the name matters — resolving the full path would land on npm-shim.js and break those checks.
• Falls back to the raw resolved path if realpath throws.
• No behavior change off macOS/fnm, or when resolution already yields a stable path.

Verification

• npx tsc --noEmit — clean.
• npx vitest run __tests__/installer-targets.test.ts — 162/162 pass.
• Manual repro on macOS + fnm: command -v codegraph → ~/.local/state/fnm_multishells/<pid>/bin/codegraph (dangles after the shell exits); after the change the written command resolves to the stable node-versions/<ver>/installation/bin/codegraph and runs.

Note on tests

Happy to add a regression test. resolveCodegraphCommand isn't exported and the suite intentionally doesn't mock execSync, so a deterministic test needs a small seam (export the function, or mock child_process). Glad to go whichever way you prefer.