[FEATURE] Idle / timeout / debounced hook event to enable customized handoffs, etc.

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

Problem

Claude Code has no hook event that fires on inactivity. Because the model is
stateless, every turn re-sends the full conversation, and the prompt cache goes
cold after the TTL (5 min on API keys, 1 hour on a subscription). If I step away
and resume a large session the next day, turn 1 re-reads the entire history at
full input rate. The expensive failure mode is forgetting a fat session is open
and resuming it cold.

I'd like to customize handoffs, cleanup, etc. after N minutes of inactivity,
while the cache is still warm, so I can have my customized /handoff ready next time I
come back to /clear and avoid being stuck with native /compact
without ever paying the cold re-read. Today this is impossible from inside Claude
Code: the hook events (SessionStart, SessionEnd, UserPromptSubmit, Stop,
StopFailure, PreCompact, ...) have no idle/timeout trigger, so it requires an
external launchd/cron watcher polling the transcript JSONL.

Why it matters

Direct token savings, prevents forgotten cold-thread re-reads, and unlocks
clean async handoff workflows (e.g. handing in-flight work to a fresh session or
another agent).

Proposed Solution

Random ideas:

• An Idle hook event that fires after a configurable inactivity threshold (e.g. idleThresholdMs in settings.json). Payload should include session_id and transcript_path so the hook can generate a handoff, auto-compact, or notify.
• Optional built-in "auto-handoff/auto-compact on idle" setting that uses (1).
• Customizable compact / auto-compact before cache cools
• Add a PreCacheCooling hook
• Bonus: a summarize-before-resume option in the --resume picker (this used to feel available and is now gone), so reopening an old session can start from a compacted summary instead of full history.

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

Interactive mode (TUI)

Use Case Example

1. I'm deep in a large Claude Code session with my custom /handoff command

configured (it writes a tailored handoff doc, not the native /compact summary).

1. I get pulled away mid-task and don't return for an hour. The prompt cache

goes cold; my session is still open but I've forgotten about it.

1. With an Idle hook firing after, say, 10 min of inactivity, Claude Code

runs my /handoff automatically while the cache is still warm, then I can
safely /clear.

1. When I come back the next day, I resume from my pre-built handoff instead of

a full cold re-read of the entire history at full input rate.

1. This saves me time and tokens because the expensive cold re-read never

happens, and I'm never stuck with the native /compact when I'd rather use my
own handoff format.

Additional Context

_No response_