DCR: PreSessionEnd and PreClear hook events

Summary

Add hook events that fire before session termination (/exit, terminal close) and before context clear (/clear).

Problem

Claude Code sessions accumulate state that needs cleanup before exit:

• Uncommitted changes that will be lost
• Unpushed commits (local work invisible to collaborators)
• Issues closed without resolution documentation
• Handoff notes not updated for session continuity
• Worktrees not cleaned up
• Background tasks still running

Current situation: Users must remember to run cleanup manually. They don't. Context is lost, work is orphaned, continuity breaks.

Proposed Solution

Add two new hook events:

PreSessionEnd

Fires before /exit or terminal close.

PreClear

Fires before /clear wipes context.

Hook Contract

{
  "hooks": {
    "PreSessionEnd": [
      {
        "type": "command",
        "command": "powershell -File cleanup.ps1"
      }
    ],
    "PreClear": [
      {
        "type": "command", 
        "command": "powershell -File save-context.ps1"
      }
    ]
  }
}

Input to hook: Session metadata (duration, tools used, files modified, git status)

Constraints:

• Must complete in <5s or timeout
• Cannot block exit (exit always succeeds even if hook fails)
• Advisory only - can display warnings but not prevent action

Use Cases

1. Warn about uncommitted/unpushed changes
2. Update session handoff notes automatically
3. Clean up temporary resources (worktrees, background tasks)
4. Audit trail logging (session end timestamp)
5. Prompt to run /session-end skill if not already run

Value Proposition

Enables "pit of success" - correct behavior by default, not by memory. Critical for methodology compliance in multi-agent workflows where session continuity matters.