security: inject X-MCP-Token on all bridge fetch sites

[D3vCrow]

Summary

• Load shared secret at module init from %APPDATA%/unity-mcp/secret (Windows) or $XDG_CONFIG_HOME/unity-mcp/secret (Unix)
• Inject X-MCP-Token via new authHeaders() helper on all 6 fetch sites in src/unity-editor-bridge.js (submitToQueue, pollQueueStatus, sendCommandLegacyMode, getQueueInfo, getTicketStatus, getProjectContext)
• Graceful degradation: if the secret file is absent, no header is sent and existing behavior is preserved — matches the plugin's EnforceToken=false default

This is the companion change to the D3vCrow fork of unity-mcp-plugin, which adds an opt-in X-MCP-Token gate on mutating verbs (POST/PUT/DELETE). Without a plugin counterpart that enforces the token, this PR is a no-op.

Test plan

• Positive path: unity_editor_ping via MCP → connected:true, Unity 6000.3.6f1, project UnityToolkit, processId 23304
• Bad Origin: curl -X POST -H 'Origin: http://attacker.example.com' ... → 403 {"error":"origin not allowed"}
• Bad token: curl -X POST -H 'X-MCP-Token: wrong' ... → 403 {"error":"token invalid"}
• Companion auto-connects to port 7891 on session restart; claude mcp list shows Connected
• Verified against Unity plugin fork commit 3c2c67a (thread-safe snapshot) — no 'GetBool can only be called from the main thread' errors

Caveats

• Token is cached at process start. Regenerating the secret in the plugin dashboard requires restarting the MCP server process.
• No new dependencies — uses Node stdlib (node:fs, node:os, node:path).

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