sandboxes: credential merging

Right now, when merging a sandbox, credentials do NOT get merged.

This is good and fine. It means I can set my sandbox to use a special dev credential, but when merging back to main, the main credential is left.

Environments mean that you shouldn't need to explicitly switch between dev and prod credentials - but as this is how users are used to working, it seems plausible that this will be done.

But there are times when this isn't useful:

• Sometimes I genuinely want to change credential (without changing environment). In this case, I need to go into main after merge and fix the workflow before it runs. Dicey.
• If I create a whole new node, chances are I want to keep the credential I've set

What I suggest is:

• Add a checkbox to the merge UI, disabled by default, which says "include credentials". This should only appear when we detect a diff on credentials.
• For new nodes, we should include the credential. Note that a name change might result in the node being considered "new", and might cause the credential to be overwritten. Feels like an edge case - a lotta ifs! - but it could also be a real problem

Start by locating the sandbox merge flow and its merge UI, then trace how credential differences and newly created nodes are currently handled. Done means an opt-in, default-disabled credential merge appears only when relevant, while new nodes retain their credentials without unintended overwrites.