[Bug] Session ID collision causes empty scrollback and "already in use" errors in background sessions

**Bug Description**
Title: Session ID collision: background sessions open with empty scrollback, new sessions fail with "already in use"
▎
▎ Environment
▎ - Claude Code v2.1.233
▎ - Windows 11 Home 10.0.26200, PowerShell
▎ - Model: Opus 5 (1M context)
▎
▎ What happened
▎ Worked normally through Aug 16, 2026. On Aug 17 the background agents view (the multi-session "claude agents" list) began failing three ways:
▎
▎ 1. Opening an existing background session from the list renders only the most recent user message — the entire prior scrollback is missing from the UI.
▎ 2. Starting a new session from the list prompt fails with: Session can't start — Session ID 24b2f818-7e64-482b-9cf4-a6ea893b057a is already ... (truncated in the UI).
▎ 3. A previously-completed row shows: exit 1 before init — Error: Ses...
▎
▎ Data is intact
▎ The transcripts are fine. ~/.claude/projects// holds all session .jsonl files at expected sizes and mtimes, including the ones that render empty. This is a UI/session-registry problem, not data loss.
▎
▎ Also noted: ~/.claude/jobs/ gained six job directories within four minutes (13:34–13:38) while these failures occurred. One of them, 24b2f818, matches the colliding session ID but has no corresponding .jsonl in the project directory.
▎
▎ Expected
▎ Opening a background session shows its full history; new sessions receive a unique ID.

**Environment Info**
- Platform: win32
- Terminal: xterm-256color
- Version: 2.1.233
- Feedback ID: d1ae0ec3-dfb4-4ef1-9b7a-957be3079033

**Errors**
```json
[]
```

Start by reproducing the failures in the background agents view on Windows with Claude Code v2.1.233, using the reported session and job directories under ~/.claude/projects// and ~/.claude/jobs/. Compare the registry/session behavior with the intact transcript files. Done means existing background sessions display their full history and new sessions receive unique IDs; the issue does not name source files or tests.

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