Local models: offer OpenCode in the tools step #60

Part of #20.

Status

• The catalog entry, the installer, and ~/.opencode/bin on the PATH landed in feat: offer Ollama and OpenCode in the tools step #131.
• Left: the second Done When item, OpenCode answering from a local model inside ally attach.
• October 27, 2026: OpenCode 2.0.12 opens inside ally attach on archbox. No model is pulled yet, so the test waits for one.

What

Add OpenCode to the tools catalog in crates/ally-core/src/tools.rs, so ally link and ally tools can install it on the Agent.

Why

OpenCode is the coding agent that runs on local models through Ollama. Inside ally attach, both run on the Agent, so no port forwarding is needed.

How

• Offer it like Claude Code and Codex: when the Client has opencode.
• Its installer, curl -fsSL https://opencode.ai/install | bash, puts it in ~/.opencode/bin. Add .opencode/bin to tools::USER_FOLDERS, or install with npm install -g opencode-ai like Codex. Pick one and say why in the pull request.
• Point it at Ollama with ollama launch opencode instead of Ally writing opencode.json.
• Ollama's docs say OpenCode needs a context length of 64k or more.

Done When

• ally tools on a Client with OpenCode offers it, shows the exact command, and installs it after one yes.
• opencode starts inside ally attach on archbox and answers from a local model.