Feature request: PreClear hook event for /clear command

Feature Request

Problem

The /clear command wipes conversation context but does not fire any hook before executing. This means:

• No opportunity to save session state before context is lost
• SessionEnd is documented to fire with reason: "clear" but doesn't actually fire (see #6428)
• Users with session-logging workflows (via hooks) lose coverage on /clear

Proposed Solution

Add a PreClear hook event that fires before /clear wipes the conversation context. This would allow users to:

• Run session loggers that read the transcript before context is gone
• Save plan/progress state
• Perform any cleanup before the reset

Input shape

Similar to SessionEnd:

{
  "session_id": "abc123",
  "transcript_path": "/Users/.../.claude/projects/.../transcript.jsonl",
  "cwd": "/Users/...",
  "hook_event_name": "PreClear"
}

Use Case

I use agent hooks on SessionEnd and PreCompact to auto-log session summaries to a daily journal. /clear is the one gap — context disappears with no hook firing. A PreClear event would close this gap and make the hook system complete for session lifecycle coverage.

Alternatives Considered

• Relying on SessionEnd — doesn't fire on /clear (#6428)
• Manual /log-session before /clear — easy to forget
• PreCompact — only covers automatic compaction, not manual /clear