[FEATURE] Support an interrupt message on stdin

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

I've built an overlay tool that runs on top of Claude CLI using --input-format stream-json in headless mode. It orchestrates multiple concurrent agents, and users need to be able to redirect agents mid-task — the same way you'd press ESC in the interactive CLI to stop the current tool and type a new instruction.

Proposed Solution

Support an interrupt message on stdin:

{"type": "interrupt"}

When received:

1. Abort the currently executing tool (same behavior as ESC)
2. Keep the process and session alive
3. Process the next queued {"type": "user", ...} message immediately

Usage pattern:
→ {"type": "interrupt"}
→ {"type": "user", "message": {"role": "user", "content": "Stop that and focus on X instead"}}

Alternative Solutions

Alternatives Considered

┌──────────────┬───────────────────────────────────┐
│ Approach │ Issue │
├──────────────┼───────────────────────────────────┤
│ Agent SDK interrupt() │ Requires API billing — can't use Claude subscription │
├──────────────┼───────────────────────────────────┤
│ SIGINT / CTRL_BREAK │ Kills the process entirely in headless mode │
├──────────────┼───────────────────────────────────┤
│ Kill + respawn -r │ Slow, fragile, loses in-flight tool state │
├──────────────┼───────────────────────────────────┤
│ Write to stdin while running │ Queued until turn ends — not a real interrupt │
└──────────────┴───────────────────────────────────┘

Priority

Medium - Would be very helpful

Feature Category

CLI commands and flags

Use Case Example

_No response_

Additional Context

_No response_