fix(gemini): use marker instead of name prefix for MCP servers

[kirillzh]

Problem

Gemini MCP server names were prefixed with ai-rules-generated- to distinguish them from user-configured servers. This breaks MCP tool calls:

• Permission: mcp__agent-task-queue__run_task
• Actual server name: ai-rules-generated-agent-task-queue
• Result: MCP call mcp__agent-task-queue__run_task() doesn't match the server

Other agents (Cursor, Roo, Claude) use ExternalMcpGenerator which preserves server names.

Solution

Use _aiRulesGenerated: true marker in the server config instead of prefixing the name:

{
  "mcpServers": {
    "agent-task-queue": {
      "command": "uvx",
      "args": ["agent-task-queue@latest"],
      "_aiRulesGenerated": true
    }
  }
}

This allows MCP tools to work correctly while still distinguishing generated from user servers for clean/update operations.

Changes

• Replace prefix_server_names() with mark_as_generated()
• Add is_generated() helper to check for marker
• Update all tests

Test plan

• All 16 Gemini tests pass
• cargo test gemini passes