feat(mcp): add JetBrains and VS Code MCP server presets

[trick77]

Summary

Adds two new presets that connect opencode to a running editor as an MCP client of the editor:

• mcp-intellij — uses the official JetBrains "MCP Server" plugin (IntelliJ IDEA, PyCharm, WebStorm, GoLand, RustRover, etc.) at http://127.0.0.1:64342/stream.
• mcp-vscode — uses the community JuehangQin.vscode-mcp-server extension at http://127.0.0.1:3000/mcp. (VS Code has no first-party equivalent of the JetBrains plugin.)

What you get

Once the editor-side plugin/extension is enabled, opencode can see what you're doing in the IDE in real time and drive it back:

• Live editor context — open files, caret position, selected text, recent files (JetBrains); diagnostics, symbol search, document outlines (both)
• Project awareness — project structure, modules, problems
• IDE actions — open files, jump to symbols, run/debug configurations (JetBrains); move/rename with refactor-aware imports (VS Code)
• Smart edits — apply changes through the editor's own edit/refactoring engine instead of raw text patching

The result: prompts like "fix the thing I'm looking at" work without copy-paste.

Mechanics

• Both presets wire a type: remote entry into opencode.json pointing at the editor's loopback endpoint (no auth — the editors bind to loopback only).
• Extends the @prompt directive with an optional 4th default field. Empty input falls back to the default; rejected on type=secret. Existing 2- and 3-field forms are untouched.
• Updates AGENTS.md (format docs) and README.md (preset table). New parser tests in test/parse-conf.test.ts.

Important: the editor-side plugin/extension is not built in. It must be installed AND switched on before either preset is useful:

• JetBrains: install the "MCP Server" plugin and enable it in Settings → Tools → MCP Server.
• VS Code: install JuehangQin.vscode-mcp-server from the Marketplace, enable it, and toggle it active via the command palette ("MCP Server: Toggle Active").
  opencode cannot reach the endpoint otherwise.

Test plan

• npm test — 47/47 pass
• npm run build clean
• mcp-intellij: install with empty input → port 64342; with 64999 → URL updated; re-install no-op; remove cleans entry
• mcp-vscode: install with empty input → port 3000, URL http://127.0.0.1:3000/mcp
• Backwards-compat: mcp-remote-add-noauth with empty input still errors
• End-to-end against a running JetBrains IDE with the MCP Server plugin
• End-to-end against a running VS Code with JuehangQin.vscode-mcp-server toggled on