feat: support OBSIDIAN_REST_API_PORT environment variable

[dominikblei]

Adds support for configuring the REST API port via the OBSIDIAN_REST_API_PORT environment variable.

This enables running multiple Obsidian vaults simultaneously, each with their own MCP server instance on different ports.

Usage:

{
  "mcpServers": {
    "vault-a": {
      "command": ".../mcp-server",
      "env": {
        "OBSIDIAN_API_KEY": "...",
        "OBSIDIAN_REST_API_PORT": "27124"
      }
    },
    "vault-b": {
      "command": ".../mcp-server",
      "env": {
        "OBSIDIAN_API_KEY": "...",
        "OBSIDIAN_REST_API_PORT": "27125"
      }
    }
  }
}

Closes #40

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