[FEATURE] Auto-delete conversation history after a configurable retention period

Problem

Claude Code stores conversation history locally, and over time this data accumulates significantly. There is currently no built-in mechanism to automatically clean up old history, which leads to ever-growing storage consumption on the user's machine. Users who rely on Claude Code daily can see this grow to hundreds of megabytes or more.

Proposed Solution

Add a configurable auto-delete setting for conversation history with preset retention periods:

• 1 month — Keep only the last 30 days of history
• 3 months — Keep only the last 90 days of history
• 6 months — Keep only the last 180 days of history
• Never (default) — Keep all history indefinitely (current behavior)

This could be exposed as a setting in the Claude Code configuration, for example:

{
  "historyRetention": "3months"
}

Expected Behavior

• A background cleanup process periodically removes conversations older than the configured retention period.
• The default remains "never delete" to preserve backward compatibility.
• Users receive no surprise deletions — the feature is strictly opt-in.

Why This Matters

• Prevents unbounded disk usage growth over time
• Gives users control over their local data lifecycle
• Aligns with privacy-conscious practices (less stale data sitting on disk)