Session history disappears when continuing a session with a new topic

Description

When a session is continued (using -c or /continue) and a new conversation topic is started, the original conversation becomes invisible in the history tab.

Steps to Reproduce

1. Start a new conversation in Claude Code (e.g., "YouTube strategy discussion")
2. Have a long conversation (300+ lines)
3. End the conversation
4. Later, continue the same session with -c or /continue
5. Start a new topic (e.g., "Playwright で MCP できないかな")
6. Check the history tab

Expected Behavior

• Both conversations should be visible in history
• OR each conversation should be treated as a separate session

Actual Behavior

• Only the second conversation's first prompt appears in history
• The original conversation (YouTube strategy) is not visible in the history tab
• The sessions-index.json contains:
• firstPrompt: Points to the second conversation's first message
• created: Timestamp of the second conversation start (not the original)
• messageCount: Only counts messages from the second conversation

Evidence

=== sessions-index.json ===
firstPrompt: "Playwrightで。MCPでできないかな"  ← Wrong (should be first conversation)
created: "2026-02-13T16:13:08.567Z"              ← Wrong (20 min after actual start)
messageCount: 10                                  ← Wrong (actual file has 300 lines)

=== Actual session file ===
Line 1-255: YouTube strategy conversation (started 15:52:32)
Line 256+: Playwright/MCP conversation (started 16:13:46)

Workaround

Manually edit ~/.claude/projects/<project>/sessions-index.json to fix the metadata.

Environment

• Claude Code version: 2.1.39 (from session file)
• OS: macOS Darwin 24.6.0
• IDE: VSCode extension

Suggested Fix

When continuing a session, the index should:

1. Preserve the original firstPrompt and created timestamp
2. OR create a new session entry for the continued conversation