enforce merge compatability between all named branches / clean merges

[sam0x17]

Description

This PR adds a GitHub Action that enforces clean merges from upstream branches into a pull request’s target branch. The workflow checks whether the relevant upstream branches can merge without conflicts before allowing the PR to be merged.

Workflow Behavior

• PR into devnet-ready → Ensures devnet-ready, devnet, testnet, and main merge cleanly.
• PR into devnet → Ensures devnet, testnet, and main merge cleanly.
• PR into testnet → Ensures testnet and main merge cleanly.
• PR into main → Ensures main merges cleanly.
• PR into any other branch → Defaults to checking all four (devnet-ready, devnet, testnet, and main).

Implementation Details

• Uses git merge --no-commit --no-ff to test merges without modifying history.
• If any merge conflict occurs, the workflow fails, preventing the PR from merging.
• Uses git merge --abort to clean up after each merge check.
• Ensures full repository history is available (fetch-depth: 0).

Why This Change?

• Prevents merge conflicts from propagating down the branch hierarchy.
• Ensures PRs are integration-ready before merging.
• Improves CI robustness and maintains a clean commit history.