Add persistent setting for fast mode default

Feature Request

Is your feature request related to a problem?

Fast mode (/fast) is currently a session-level toggle only. Every new session starts with fast mode off, requiring me to run /fast at the beginning of each session. This is a minor but repetitive friction point.

Describe the solution you'd like

Add a persistent setting (e.g., fastMode: true in ~/.claude/settings.json) that enables fast mode by default for all new sessions. Something like:

{
  "preferences": {
    "fastMode": true
  }
}

Or alternatively, a CLI flag like claude --fast that could be aliased.

Describe alternatives you've considered

• Running /fast manually at the start of each session (current workaround)
• Shell alias that sends /fast after launch (fragile)

Additional context

Fast mode uses the same Opus model with faster output — there's no quality tradeoff for many use cases, so having it as a persistent default would be valuable for users who prefer it.