fix(bash): make description optional for tool calls

[ymylive]

Fixes #13146

Summary

• Make description optional in BashTool input schema
• Add a fallback description (Run shell command) when callers omit it
• Add regression test to verify bash.execute({ command }) works without description

Why

Some clients/tool-call paths invoke bash with only command. Requiring description causes schema validation errors and makes bash effectively unusable in those flows.

Scope

• packages/opencode/src/tool/bash.ts
• packages/opencode/test/tool/bash.test.ts

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[SeanThomasWilliams]

We're seeing this issue in production with NVIDIA Nemotron-3-Nano (30B-A3B, AWQ variant) served via vLLM on DGX Spark hardware. The model was trained without the description parameter and omits it in ~78% of multi-turn bash tool calls, causing validation failures.

Validation evidence from our testing:

• Single-turn bash calls: 100% include description when prompted via system message, but this breaks down in multi-turn agentic workflows where context depth increases.
• Multi-turn agentic exercises (Bug Hunt, TDD, Spec-to-Code): ~22% bash call success rate without this fix.
• Temperature sweep (0.0-0.6): Does not affect schema adherence — the issue is structural, not sampling-related.
• Cross-model: This affects multiple open-weight models (confirmed in issue discussion), not just Nemotron.

Our workaround: Custom tool override via .opencode/tools/bash.ts that makes description optional with a fallback string. Works but shouldn't be necessary for a core tool parameter that many models weren't trained with.

This fix would significantly improve opencode's compatibility with open-weight models. The approach in this PR (optional schema + fallback) is exactly right.