[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.