Add opencode skill + MCP server installation to installers

[aniongithub]

Summary

Adds opencode (https://opencode.ai) as a first-class target in both installers, alongside the existing GitHub Copilot CLI, Claude Code, VS Code, and Cursor support.

Why

opencode is gaining traction as an MCP-compatible coding agent. Until now, users had to hand-edit ~/.config/opencode/opencode.json after running the installer. This PR makes it just work.

What changed

opencode reads MCP configuration from ~/.config/opencode/opencode.json using a distinct schema from the mcpServers convention used by every other client we support:

• top-level key is mcp (not mcpServers)
• requires "type": "local"
• command is a single array (binary + args combined) — not separate command string + args array
• explicit "enabled": true

That meant a dedicated config function in each installer rather than reusing the existing configure_mcp_client / Set-McpConfig helpers.

install.sh

• Add ~/.config/opencode/skills/devcontainer-mcp to SKILL_DIRS so SKILL.md lands at opencode's documented global skills path.
• New configure_opencode_mcp function. Creates the file if missing; otherwise merges via python3 — preserves any existing $schema, model, sibling MCP entries, and other settings.

install.ps1

• Mirror the skill directory addition.
• New Set-OpencodeMcpConfig that writes the WSL-bridge form: command: ["wsl", "~/.local/bin/devcontainer-mcp", "serve"].

README.md

• List opencode in the "Works with" badge line.

Scope note: host-protection hooks

opencode does have a hook mechanism, but it works very differently from Claude Code's PreToolUse / SessionStart (JSON pointing at a shell script). opencode uses a JS/TS plugin system (docs) where plugins live in ~/.config/opencode/plugins/ and subscribe to events like tool.execute.before and session.created.

Wiring our devcontainer-guard.sh and devcontainer-skill-loader.sh into that model requires writing a thin JS plugin that spawns the existing bash hooks — out of scope for this PR. Tracked as a follow-up issue.

Verification

Spun up the project's own devcontainer (debian-bookworm + rust + node + docker-in-docker) and ran install.sh inside. All checks pass:

Test	Result
Fresh-install creates ~/.config/opencode/opencode.json with valid schema	✅
SKILL.md placed at ~/.config/opencode/skills/devcontainer-mcp/	✅
Schema validated: $schema, mcp.devcontainer-mcp.{type,command,enabled} correct	✅
Idempotent rerun → "opencode — already configured"	✅
Merge into pre-existing config preserves $schema, model, and a sibling MCP entry	✅
Binary at the configured command[0] launches correctly (devcontainer-mcp --help succeeds)	✅

Sample of the file the installer produces:

{
  "$schema": "https://opencode.ai/config.json",
  "mcp": {
    "devcontainer-mcp": {
      "type": "local",
      "command": ["/home/vscode/.local/bin/devcontainer-mcp", "serve"],
      "enabled": true
    }
  }
}

[aniongithub]

Follow-up issue for opencode host-protection hooks: #23