[BUG] Session History panel empty across all projects after extension update to 2.1.270 — data intact on disk, fixed by downgrading to 2.1.266

### Preflight Checklist
- [x] I have searched existing issues and this hasn't been reported yet (as a fresh, non-stale report)
- [x] This is a single bug report
- [x] I am using the latest version of Claude Code

### What's Wrong?

After the Claude Code VS Code extension auto-updated, the Session History panel showed zero past sessions across ALL projects (not just one) — only the currently active session appeared. This happened even though the actual conversation transcripts on disk were completely intact.

Confirmed root cause: version regression in the VS Code extension between 2.1.266 (working) and 2.1.270 (broken).

Investigation steps taken before finding the fix:
- Verified all `.jsonl` transcript files were present and untouched under `~/.claude/projects//`
- Checked `hiddenSessionIds` in the extension's VS Code globalState (`state.vscdb`, key `Anthropic.claude-code`) — it was empty, so archiving was not the cause
- A full VS Code window reload did not fix it
- Searching for a known project name in the Session History search box returned "No sessions found"
- Compared a `state.vscdb.backup` from before the issue against the current file — the only meaningful difference was the recorded extension version: 2.1.266 vs 2.1.270

### What Should Happen?

Session history should keep working across extension updates. If a storage/index format change is needed, it should migrate existing sessions rather than silently dropping them from the panel.

### Error Messages/Logs

None found in the extension host log (`Claude VSCode.log`) — the failure is silent.

### Steps to Reproduce

1. Have existing Claude Code session history across multiple projects on the VS Code extension
2. Let the extension auto-update from 2.1.266 to 2.1.270
3. Open the Session History panel — only the current session appears, all prior sessions are gone from the list (though the `.jsonl` files remain on disk)

### Claude Model
Not sure / Multiple models

### Is this a regression?
Yes, this worked in a previous version

### Last Working Version
2.1.266 (VS Code extension version)

### Claude Code Version
2.1.237 (Claude Code) — CLI version. The bug is in the VS Code extension specifically: broken on extension v2.1.270, fixed on v2.1.266.

### Platform
Anthropic API (connected via Claude.ai Pro/Max subscription OAuth, not a standalone API key)

### Operating System
Windows

### Terminal/Shell
VS Code integrated terminal

### Additional Information

This appears to be the same underlying issue reported in #64082, #55066, and #78578, all closed by inactivity without a maintainer fix. This report adds a confirmed working/broken version pair (2.1.266 / 2.1.270) for the VS Code extension specifically, and rules out the archiving (`hiddenSessionIds`) mechanism as the cause, which should help narrow down the actual regression in the extension's session-discovery code.

Start by inspecting the VS Code extension's session-discovery code and how it reads .jsonl files under ~/.claude/projects//. Compare behavior between extension versions 2.1.266 and 2.1.270, using the VS Code globalState state.vscdb and Claude VSCode.log for evidence. Done means existing sessions from multiple projects appear after updating, without requiring a downgrade or losing data.

Rédigé par le modèle d'indexation à partir du texte de l'issue.