refactor(run): report the assumed platform per environment

[hunger]

The pixi run platform decision moved into the per-task loop (#6507), so the up-front base-environment logging no longer matched what actually ran: a bare pixi run would log "assuming platform linux-64" for the default environment while the task ran in a sibling at a richer platform.

Emit the "assuming platform" info per environment, keyed on the task's run environment and its resolved platform, and drop the verbose "Run-platform decision" debug line -- it only described the base environment's inputs and is no longer needed.

How Has This Been Tested?

By checking the logging output

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Tools: Claude

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[hunger]

Based on your comment to #6507: Yes, much of this can probably go now. Here is the cleanup for that.

I did not want to do that in the fix PR as it felt like refactoring and not like a fix, but that distinction is unnecessary when you release straight from main anyway :-)

[hunger]

@wolfv Considering your more ambitious cleanup PR #6509 changes behavior in ways we do not want: Shall we merge this one? It is not changing behavior:-)