[BUG] --continue resumes from wrong point after parallel Agent execution

Description

When using --continue to resume a session that had multiple Agents launched in parallel, the session rolls back to the point where the Agents were launched, ignoring all subsequent conversation.

Steps to Reproduce

1. Use a session over multiple days with --continue
2. Launch 8+ Agents in parallel (e.g., reading files from multiple projects)
3. Continue working after Agents complete
4. End session, then resume with --continue
5. Session resumes from step 2, not step 3

Root Cause (from JSONL analysis)

• file-history-snapshot entries written by parallel Agents all carry the timestamp of the Agent launch moment
• These snapshots accumulate at the tail of the JSONL file
• On --continue, the tail snapshots point to the Agent launch time, causing the session to resume from that point
• The final snapshot had an orphaned messageId (not matching any UUID in the JSONL), so it was likely skipped

Environment

• Claude Code v2.1.100
• Platform: WSL2 Ubuntu on Windows
• Session duration: 8 days (with regular /compact)
• Session JSONL size: 15MB (5101 lines)
• file-history-snapshot entries: 981 (26% of file)
• 48 total Agent invocations, including 8 launched within 10 seconds

Key Data

• Tail snapshots (lines 5078-5099): all timestamped 2026-04-09T20:56~21:24
• Last user message: line 5074, 2026-04-10T12:38
• Last snapshot (line 5101): orphaned messageId with no matching UUID
• parentUuid chain: intact (0 breaks)
• Compact executed 6 times (not zero) — compaction does not clear snapshots

Expected Behavior

--continue should resume from the last user/assistant message, regardless of parallel Agent snapshot timestamps.

Actual Behavior

--continue resumes from the Agent parallel launch point (April 9), skipping all work done on April 10. This creates an infinite rollback loop — every --continue returns to the same point.