fix(mcp): share one panel key across an install's stdio servers

Fixes bilawalsidhu/gods-eye-view#927. Each stdio MCP server made its own
random panel key, and panel_request accepts only the key in the panel
page. A client that starts two server processes for one connection and
reads the page from one while sending the page's requests to the other
(seen in Claude Desktop on Windows) therefore always got "Only the God's
Eye View panel may make this request".

The stdio entry now passes a key shared by the install, created once in
.gev-cache/mcp-panel-key. The file is staged 0600 and published with
link(), so processes starting together agree on one key and never read a
half-written one. If it cannot be stored, the process keeps a key of its
own, as before. createLocalMcpServer still makes a fresh key unless given
one, so in-process servers (the /mcp HTTP plugin, tests) are unchanged.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

server/mcp/server.js

@@ -27,14 +27,16 @@ const INSTRUCTIONS =
 export function createLocalMcpServer({
   apiBase = DEFAULT_API_BASE,
   fetchImpl,
+  panelKey = randomBytes(32).toString('base64url'),
 } = {}) {
   const { version } = JSON.parse(
     readFileSync(new URL('../../package.json', import.meta.url), 'utf8'),
   );
-  // Each server makes a new key for its panel. The panel's page carries it
-  // and panel_request requires it; any client may read the page, so it keeps
-  // the tool from clients that only list it, and is not access control.
-  const panelKey = randomBytes(32).toString('base64url');
+  // Unless given one, each server makes a new key for its panel. The panel's
+  // page carries it and panel_request requires it; any client may read the
+  // page, so it keeps the tool from clients that only list it, and is not
+  // access control. The stdio entry passes the install's shared key
+  // (panelKey.js), so separate processes of one client agree.
   return createMcpServer({
     catalog: catalogForSurface(
       composeCatalog({