Add setup-pspdev and setup-devkitpro composite actions; enable gamecube-iso

Extracts toolchain setup into reusable composite actions under
.github/actions/. Comments out build-psp, adds build-gamecube-iso
job using the new setup-devkitpro action directly on the runner.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/actions/setup-pspdev/action.yml

@@ -0,0 +1,20 @@
+name: Setup pspdev
+description: Install the pspdev PSP toolchain on an Ubuntu runner
+
+runs:
+  using: composite
+  steps:
+    - name: Install dependencies
+      shell: bash
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y cmake python3 python3-pip python3-dotenv
+    - name: Install pspdev toolchain
+      shell: bash
+      run: |
+        wget -q \
+          https://github.com/pspdev/pspdev/releases/latest/download/pspdev-ubuntu-latest-x86_64.tar.gz \
+          -O /tmp/pspdev.tar.gz
+        sudo tar -xzf /tmp/pspdev.tar.gz -C /usr/local
+        echo "PSPDEV=/usr/local/pspdev" >> $GITHUB_ENV
+        echo "/usr/local/pspdev/bin" >> $GITHUB_PATH