OpenCode's free tier now returns HTTP 403 for anonymous traffic outside
the OpenCode client, so the built-in keyless provider is dead weight:
- drop the opencode-free provider row, aliases (free/opencode_free), model
catalog, keyless runtime ladder rung, header wiring, and cached slugs
- delete the model-providers/opencode-free plugin
- update tests and the compat manifest for the removed symbols
- keep a migration hint in auth.py so users who had it configured see a
clear error naming the removal
Existing opencode-free configs can move to opencode-zen (pay-as-you-go)
or opencode-go (flat subscription).
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