Feature Request: Add preCompact hook event for context refresh before token compaction

[cephalin]

Summary

Copilot CLI hooks are missing a preCompact event type that exists in Claude Code. This creates a gap for workflows that need to refresh context before token compaction (like Gas Town projects).

Background

Claude Code's .claude/settings.json supports a PreCompact hook that runs before context compaction:

{
  "hooks": {
    "PreCompact": [
      {
        "type": "command",
        "command": "gt prime --hook"
      }
    ]
  }
}

This is critical for projects like Gas Town where agents (mayor, deacon, refinery, witness, crew, polecats) need to:

1. Inject fresh context via gt prime --hook on session start (sessionStart)
2. Refresh that context right before compaction (PreCompact) to ensure accurate state before tokens are compressed

Problem

Copilot CLI currently supports:

• sessionStart
• sessionEnd
• userPromptSubmitted
• preToolUse
• postToolUse
• errorOccurred

But NOT preCompact. This means agents running in Copilot CLI have stale context after compaction, which breaks multi-hour workflows.

Request

Add support for a preCompact hook event (or expose the internal compaction_start event) in Copilot CLI's .github/hooks/*.json configuration format, matching Claude Code's behavior.

Impact

• Enables Gas Town and similar systems to work reliably with Copilot CLI for long sessions
• Maintains parity with Claude Code's hook ecosystem