Remove experimental feature flag for planning

[jerop]

Before - users have to opt in (defaults to false)

{
  "experimental": {
    "plan": true # defaults to false
  }
}

After - users have to opt out (defaults to true)

{
  "experimental": {
    "plan": false # defaults to true
  }
}

or

{
  "general": {
    "plan": {
      "enable": "true", # defaults to true
      "directory": ".gemini/plans"
    }
  }
}