fix(ci): add workflow_dispatch trigger and correct arm64 runner label

[maxamillion]

Summary

• Adds workflow_dispatch trigger for on-demand manual runs
• Fixes arm64 runner label from ubuntu-latest-arm64 (invalid) to ubuntu-24.04-arm (correct per GitHub docs)

Changes

File: .github/workflows/midstream-container-build.yml

1. workflow_dispatch — enables manual triggering via GitHub UI or gh workflow run
2. arm64 runner label — ubuntu-latest-arm64 does not exist as a GitHub-hosted runner label; the correct label is ubuntu-24.04-arm. This was causing arm64 jobs to queue indefinitely with no matching runner.

Usage

After merging, trigger a manual run with:

gh workflow run "Midstream Container Build" --repo LobsterTrap/OpenShell --ref midstream

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