Add OpenCode runner

[that-github-user]

Summary

Add a runner for OpenCode to enable multi-tool ensembles.

The Runner interface in src/runners/base.ts is already designed for this — implement available() and run() for OpenCode's CLI.

Why

thinktank currently only supports Claude Code. The research consistently shows that multi-model/multi-tool ensembles capture more diversity than same-model sampling. Adding OpenCode (which supports multiple LLM providers) would be the first step toward heterogeneous ensembles.

References

• Runner interface: src/runners/base.ts
• Registry: src/runners/registry.ts
• Existing Claude Code runner: src/runners/claude-code.ts