[Feature Request] Preserve session history during Claude Code upgrades

Bug Description
Title:
Auto-preserve session history files during Claude Code upgrade

Problem:
When Claude Code upgrades to a newer version, existing session
JSONL files under C:\Users\<user>\.claude\projects\ may be lost
or become inaccessible if the installer overwrites the user data
directory.

Session files are small (2-5 MB each) but represent significant
investment — long technical investigations with hundreds of
thousands of tokens of context that cannot be cheaply recreated.

Requested behavior:

1. Before upgrade: warn user that session data will be preserved
2. During upgrade: never delete or overwrite ~/.claude/projects/
3. After upgrade: verify all existing sessions are still resumable
4. Option in settings: configure custom backup path for sessions
5. Bonus: built-in export command

claude --export-session <id> --format markdown

Why:
Storage is cheap (KB per session).
Tokens to recreate context are expensive (time + API cost).
Users should never lose investigation history from an app upgrade.

Platform: Windows / macOS / Linux
Version affected: all

Environment Info

• Platform: win32
• Terminal: windows-terminal
• Version: 2.1.63
• Feedback ID: 3cd6764b-48d0-4d61-9573-55cb2e62f31f

Errors

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