Add opencode model variant customization

[nhojb]

Opencode does not currently support setting a persistent model variant via the opencode config json - or at least it does not work when running via acp. So this PR adds:

1. Support for setting the default-thought-level-id via agent-shell-make-agent-config
2. A new agent-shell-opencode-default-model-variant defcustom

Note: if the variant is not supported by the current model, then the user will see an error when loading the session

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[xenodium]

Thanks for the PR!

I'm wondering if you'd be keen to steer the PR towards a more generic mechanism? Instead of adding agent-shell-opencode-default-model-variant, we add something like:

(defcustom agent-shell-opencode-default-config-options
  '(("model"         . "anthropic/claude-opus-4-5")
    ("thought_level" . "high")
    ("mode"          . "plan"))
  "Default OpenCode config options, applied at session start.
Keys are config-option categories advertised by the agent; see
the \"Available config options\" section in your shell."
  :type '(alist :key-type string :value-type string))

I know we haven't done this before in project, but it would enable us to set precedence for moving towards a more scalable mechanism for users to set whatever option is supported by their agent without requiring additional agent-shell changes.

To support this path, I've just submitted 95c700a which enables users to know exactly what is and isn't supported by their agents.

[nhojb]

I'm wondering if you'd be keen to steer the PR towards a more generic mechanism?

Sounds good to me. I will also mark the old agent-shell-opencode-default-model-id as deprecated and fallback if the new config does not exist.