Allow per-session usage caps (percentage of overall limit)

Problem

With the Max subscription's generous limits, there's a strong incentive to automate workflows — scheduled agents, cron jobs, background monitoring, etc. However, the fear of hitting the 5-hour rolling or weekly usage cap prevents users from fully leveraging automation.

The core tension: automation is most valuable when it runs unattended, but unattended usage can silently consume the budget needed for interactive work.

Current workarounds are all bad:

• Only automate at night → weekly limit still drains
• Don't automate at all → leaves value on the table
• Automate and hope → risk being blocked from manual work when you need it most

Proposed Solution

Allow users to set a per-session (or per-agent) usage cap as a percentage of their remaining usage limit.

Example API:

claude --max-usage 50%

or in settings:

{
  "maxSessionUsage": "50%"
}

This would mean: "this session may use at most 50% of my current remaining limit — reserve the rest for other sessions."

Use Cases

1. Background automation: Run a scheduled agent with --max-usage 10% so it can never starve interactive sessions
2. Exploration vs. production: Cap a speculative research session at 30% so you still have room for real work
3. Team lead pattern: A lead orchestrating 5 worker agents could cap the whole team at 60%, keeping 40% for manual oversight

Why This Matters

The 20x subscription tier is designed for power users who want to automate. But without usage governance tools, the rational behavior is to under-utilize automation out of caution. Per-session caps would unlock the full potential of automation features (cron, scheduled agents, background tasks) by giving users confidence that they won't lock themselves out of interactive work.