fix: remove non-existent autoCheckpoint config from docs

[bluzername]

Problem

The checkpoints documentation (chapter 08) references an autoCheckpoint setting that does not actually exist in Claude Code:

{
  "autoCheckpoint": true
}

Users following this guide will add this setting and wonder why it doesnt do anything. Claude Code creates checkpoints automatically on every user prompt and there is no way to disable this.

What I changed

08-checkpoints/README.md:

• Removed the autoCheckpoint JSON config block and its description
• Replaced with accurate text: checkpoints are built-in and cannot be disabled
• Mentioned cleanupPeriodDays which IS a real setting
• Updated troubleshooting section to remove reference to checking autoCheckpoint

.claude/skills/lesson-quiz/references/question-bank.md:

• Q4: Removed autoCheckpoint from answer option D (replaced with retention period reference)
• Q8: Changed correct answer from B ("set autoCheckpoint: false") to D ("cannot be disabled") since the setting doesnt exist
• Updated explanation to match reality

Why

The autoCheckpoint setting does not exist in Claude Code's settings schema. Checkpoints are always created automatically and there is no toggle for this. Having this in the docs confuses users who try to configure it.

Closes #18