Add required AMI to demo workflows

Now that ec2_image_id is required (no hardcoded fallback), the demo
workflows need to explicitly specify an AMI. Added Ubuntu 24.04 LTS
x86_64 AMI (ami-0e86e20dae9224db8) to:

- demo-archs.yml (x86 job)
- demo-multi-instance.yml
- demo-job-seq.yml
- demo-matrix-wide.yml
- demo-multi-job.yml

The GPU demos and ARM job already had AMIs specified.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: ryan-williams

.github/workflows/demo-archs.yml

@@ -12,7 +12,7 @@ jobs:
     uses: ./.github/workflows/runner.yml
     with:
       ec2_instance_type: t3.medium
-      # Ubuntu 24.04 LTS x86_64 (default AMI)
+      ec2_image_id: ami-0e86e20dae9224db8  # Ubuntu 24.04 LTS x86_64 (us-east-1)
     secrets: inherit
   arm:
     name: Launch ARM

.github/workflows/demo-job-seq.yml

@@ -23,6 +23,8 @@ jobs:
   ec2:
     uses: ./.github/workflows/runner.yml
     secrets: inherit
+    with:
+      ec2_image_id: ami-0e86e20dae9224db8  # Ubuntu 24.04 LTS x86_64 (us-east-1)
 
   prepare:
     needs: ec2

.github/workflows/demo-matrix-wide.yml

@@ -9,6 +9,8 @@ jobs:
   ec2:
     uses: ./.github/workflows/runner.yml
     secrets: inherit
+    with:
+      ec2_image_id: ami-0e86e20dae9224db8  # Ubuntu 24.04 LTS x86_64 (us-east-1)
   test-matrix:
     needs: ec2
     runs-on: ${{ needs.ec2.outputs.id }}

.github/workflows/demo-multi-instance.yml

@@ -25,6 +25,7 @@ jobs:
     secrets: inherit
     with:
       instance_count: ${{ inputs.instance_count }}
+      ec2_image_id: ami-0e86e20dae9224db8  # Ubuntu 24.04 LTS x86_64 (us-east-1)
 
   parallel-jobs:
     needs: ec2

.github/workflows/demo-multi-job.yml

@@ -14,6 +14,7 @@ jobs:
     secrets: inherit
     with:
       instance_count: "2"
+      ec2_image_id: ami-0e86e20dae9224db8  # Ubuntu 24.04 LTS x86_64 (us-east-1)
 
   # First job type - runs on first instance
   build-job: