feat: make systemDefaultModel optional for OpenCode fallback

[justsisyphus]

Summary

Make systemDefaultModel optional throughout the plugin, allowing OpenCode's built-in model fallback to work when user doesn't explicitly configure a default model.

Problem

Previously, the plugin threw an error when no default model was configured:

oh-my-opencode requires a default model.

Add this to ~/.config/opencode/opencode.jsonc:

  "model": "anthropic/claude-sonnet-4-5"

However, OpenCode has its own fallback mechanism that automatically picks the best available model when none is specified. The plugin was blocking this behavior unnecessarily.

Changes

Core Changes

• model-resolver.ts: Made systemDefaultModel optional in types, returns undefined when no model can be resolved
• config-handler.ts: Removed the entire throw block (lines 108-141) that required a default model
• delegate-task/tools.ts: Removed the guard that blocked category delegation without systemDefaultModel
• agents/utils.ts: Removed throw, agents are created only if model resolution succeeds
• agents/atlas.ts: Made ctx.model optional

Test Updates

• Added tests for optional systemDefaultModel scenarios
• Updated existing tests to use non-null assertions where appropriate

Verification

• bun run typecheck passes
• All modified test files pass (50+ tests)
• Backward compatible: if user specifies model, it's used; if not, OpenCode decides

Closes #1129

---

Summary by cubic

Make the default model optional so OpenCode can use its own fallback when a user doesn’t configure one. This removes hard errors and lets agents and category delegation run without a pre-set model. Closes #1129.

• New Features
  • Made systemDefaultModel optional in the resolver; returns undefined when no override or fallback is found so OpenCode can pick the model.
  • Removed throws in config-handler, agents/utils, atlas, and delegate-task; Sisyphus and Atlas now use fallback chains and proceed without a default model.
  • Added tests for optional model scenarios and bumped optional oh-my-opencode binaries to 3.1.0.

^{Written for commit 840a76d. Summary will update on new commits.}

[cubic-dev-ai]

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Confidence score: 5/5

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