OpenCode native plugin for auto-discovery of installed harnesses

[johnnyhuy]

Why

src/main/installer/index.ts discovers harnesses via which() then falls back to the npm-global root. On macOS when the user installed via Homebrew the claude binary lives in /opt/homebrew/bin — already on PATH for most users but not in npm-global. On Linux with NVM-flipping users, the resolution gets fragile.

OpenCode reads ~/.config/opencode/opencode.json for everything: providers, plugins, MCP servers. It supports installing community plugins via the plugin[] array. A small OpenCode plugin installed via opencode plugin install @hoist/discover could run at OpenCode's startup and:

1. Read /usr/bin/env + ~/.bashrc/~/.zshrc paths to discover installed harness binaries
2. Compare against the hoist catalog (src/main/providers/harnesses.ts shape)
3. Report back to hoist via a local Unix socket or ~/.cache/hoist/discovery.json

This is an OpenCode-side affordance, not hoist-side — but it's the cleanest way to feed accurate detection back into the GUI's "Installed · v2.1.204" cards.

Scope

• New repo under packages/opencode-plugin-hoist/ (or just in src/opencode-plugin/ until we publish).
• Small plugin with no deps: spawns which for each catalog binary, writes ~/.cache/hoist/discovery.json with { path, version } per tool.
• installer/discoverInstalled() reads that file first (if newer than 5 minutes), then falls back to the current which + npm-global probe.
• Renderer keeps showing the discovered status without a UI change.
• CLI hoist install/hoist list picks it up via the same shared discoverInstalled.

Acceptance criteria

• Installing the OpenCode plugin populates ~/.cache/hoist/discovery.json within 200 ms of OpenCode startup.
• If plugin isn't installed, behavior is identical to today (which + npm-global).
• discoverInstalled reads the cache file when present and fresh, otherwise falls back.
• Plugin is small (< 200 lines of TS, no runtime deps).
• Documented install path: opencode plugin install @hoist/discover (or equivalent local-path install during dev).

Non-goals

• Real-time signalling (file mtime is fine for v1)
• Auto-install from the plugin (that's hoist's job — keep the boundary clean)
• Cross-platform notification (linux/macOS only for v1)