feat(prd-1): implement M1 - platform detection and action.yml modifications

- Add src/platform/detector.ts with 3-tier detection (explicit input,
  FORGEJO env var, GITHUB_SERVER_URL heuristic)
- Add 'platform' input to action.yml (auto/github/forgejo)
- Add 'Detect Platform' shell step as first composite action step
- Split bun installation: oven-sh/setup-bun for GitHub, curl fallback
  for Forgejo runners
- Condition 'Revoke app token' step to skip on Forgejo
- Pass resolved PLATFORM env var to run step
- Add 14 unit tests covering all detection paths

Progress: M1 complete (1/9 milestones) - M2 and M5 now unblocked

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

action.yml

@@ -138,6 +138,12 @@ inputs:
     required: false
     default: ""
 
+  # Platform configuration
+  platform:
+    description: "Platform to run on: 'auto' (default auto-detects), 'github', or 'forgejo'"
+    required: false
+    default: "auto"
+
 outputs:
   execution_file:
     description: "Path to the Claude Code execution output file"
@@ -158,13 +164,43 @@ outputs:
 runs:
   using: "composite"
   steps:
+    - name: Detect Platform
+      id: detect-platform
+      shell: bash
+      run: |
+        if [ "${{ inputs.platform }}" = "forgejo" ]; then
+          DETECTED="forgejo"
+        elif [ "${{ inputs.platform }}" = "github" ]; then
+          DETECTED="github"
+        elif [ "${FORGEJO}" = "true" ]; then
+          DETECTED="forgejo"
+        elif [ -n "${GITHUB_SERVER_URL}" ] && echo "${GITHUB_SERVER_URL}" | grep -qv "github.com"; then
+          DETECTED="forgejo"
+        else
+          DETECTED="github"
+        fi
+        echo "platform=${DETECTED}" >> "$GITHUB_OUTPUT"
+        echo "Detected platform: ${DETECTED}"
+
     - name: Install Bun
-      if: inputs.path_to_bun_executable == ''
+      if: inputs.path_to_bun_executable == '' && steps.detect-platform.outputs.platform != 'forgejo'
       uses: oven-sh/setup-bun@3d267786b128fe76c2f16a390aa2448b815359f3 # https://github.com/oven-sh/setup-bun/releases/tag/v2.1.2
       with:
         bun-version: 1.3.6
         token: ${{ inputs.github_token || github.token }}
 
+    - name: Install Bun (Forgejo)
+      if: inputs.path_to_bun_executable == '' && steps.detect-platform.outputs.platform == 'forgejo'
+      shell: bash
+      run: |
+        if command -v bun &> /dev/null; then
+          echo "Bun is already installed: $(bun --version)"
+        else
+          echo "Installing Bun via curl..."
+          curl -fsSL https://bun.sh/install | bash
+          echo "$HOME/.bun/bin" >> "$GITHUB_PATH"
+        fi
+
     - name: Setup Custom Bun Path
       if: inputs.path_to_bun_executable != ''
       shell: bash
@@ -188,6 +224,9 @@ runs:
       run: |
         bun run ${GITHUB_ACTION_PATH}/src/entrypoints/run.ts
       env:
+        # Platform detection
+        PLATFORM: ${{ steps.detect-platform.outputs.platform }}
+
         # Prepare inputs
         MODE: ${{ inputs.mode }}
         PROMPT: ${{ inputs.prompt }}
@@ -283,7 +322,7 @@ runs:
         bun run ${GITHUB_ACTION_PATH}/src/entrypoints/cleanup-ssh-signing.ts
 
     - name: Revoke app token
-      if: always() && inputs.github_token == '' && steps.run.outputs.skipped_due_to_workflow_validation_mismatch != 'true'
+      if: always() && inputs.github_token == '' && steps.run.outputs.skipped_due_to_workflow_validation_mismatch != 'true' && steps.detect-platform.outputs.platform != 'forgejo'
       shell: bash
       run: |
         curl -L \