[FEATURE] Archive (hide-but-keep) chat sessions in VS Code sidebar

Problem

The Claude Code session list in the VS Code sidebar shows every saved
session for a project. After ~20 sessions in one project, the list
becomes hard to scan for live work-in-progress.

Today the only options are:

• Resume (keep visible)
• Rename (still visible, just relabeled)
• Remove (permanent delete)

Renaming to a [DONE] prefix is the current workaround but the
finished sessions still take up sidebar real estate.

Proposed feature

Add an archive action (Gmail-style hide-but-keep):

• Archived sessions disappear from the default sidebar list
• A "Show archived" toggle or filter surfaces them when needed
• Archived .jsonl files remain on disk and remain resumable

Why this matters

Long-running projects accumulate dozens of sessions, many of which
become reference material rather than active work. Today there's no
middle ground between "keep cluttering the list" and "delete
forever." Archive solves this without losing history.

Environment

• Claude Code VS Code extension
• Windows 11