[Feature]: Seed MacPorts /opt/local on GUI-launched terminal PATH

## Summary

GUI-launched Orca terminals inherit a minimal PATH. Homebrew prefixes are already seeded; MacPorts is not.

Intel Macs that moved to MacPorts after Homebrew dropped official Intel support install tools in `/opt/local/bin`. Those binaries are invisible in Orca until the user aliases them into `~/.local/bin`.

## Expected

If MacPorts is installed at the default prefix (`/opt/local/bin/port`), packaged terminals and the CLI install-dir fallback should see `/opt/local/bin` the same way they already see Homebrew.

Machines without MacPorts should not get a phantom `/opt/local` on PATH.

## Notes

Default pkg installs land at `/opt/local`. A custom prefix under `/usr/local` is already covered by the existing `/usr/local/bin` seed.

Locate the existing Homebrew PATH seeding and the CLI install-dir fallback. Add coverage for the default MacPorts prefix so /opt/local/bin appears only when /opt/local/bin/port is installed, then verify that packaged terminals and the CLI see it while machines without MacPorts do not.

Written by the indexing model from the issue text.