Skip plugin auto-start in headless batchmode (fix CI / -runTests)

[nwl9009]

Problem

The Editor plugin auto-starts and connects to the local MCP server on every domain load via Startup ([InitializeOnLoad] → UnityMcpPluginEditor.Instance.BuildMcpPluginIfNeeded() in Editor/Scripts/Startup.cs). This also fires under headless -batchmode -runTests. When the package is committed to a project whose CI runs Unity headless:

1. The headless instance connects to the same local MCP server as the developer's interactive Editor on that machine and forcibly disconnects the interactive client — log: McpManagerClientHub: Server forcefully disconnected this plugin. Reason: Authorization failed. Token may be missing, invalid, or revoked.
2. In a clean CI checkout there's no token, so the plugin logs an [Error] from a background SignalR connection callback. Unity's Test Framework converts that stray error into a test failure, so every PlayMode test goes red regardless of its own assertions. It can't be suppressed with LogAssert.ignoreFailingMessages because the log isn't raised on the test's main-thread scope.

Fix

Return early from Startup's static constructor when Application.isBatchMode, so the plugin neither builds nor connects during headless runs. Intentional headless use (e.g. running the MCP server in CI on purpose) is preserved via an opt-in environment variable UNITY_MCP_BATCHMODE=1.

if (Application.isBatchMode && System.Environment.GetEnvironmentVariable("UNITY_MCP_BATCHMODE") != "1")
    return;

Notes

• Minimal, behavior-preserving for interactive use (the common path is unaffected).
• Workaround without this change: strip the package from the CI worktree before -runTests. A built-in gate removes that need.

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