Allow renaming or moving a project folder while persisting history for Copilot chats (re-file of #313970)

This supersedes #313970, which was closed by the backlog-candidate process after it did not reach 20 community upvotes in 60 days. The need has not gone away, and if anything has grown with agentic workflows, so I am re-filing with the additional feedback that accumulated there.

## Problem

As a user, I often don't know upfront where a project folder will finally live. I might start working in a folder on the Desktop and move it later, or restructure existing folders. Currently, moving or renaming a project folder loses the association to its Copilot chat/session history (related: local history #152415, workspace-scoped storage #257143).

Losing session history this way actively discourages healthy project restructuring, and with more and more agentic coding (including the Agents window), long-lived session history attached to a project keeps getting more valuable.

## What would help

1. **Supported rename/move**: a context-menu action or command ("Move/Rename project folder") that relocates the folder and transparently re-maps existing Copilot chat sessions to the new location.
2. **Folder identity instead of absolute paths**: identify a project by content fingerprint or an ID stored inside the folder, so sessions can be re-associated after a move, and "orphaned" sessions can be offered for re-linking.
3. **Workspace-detached history** (from @Metadorius on #313970): chats stored independently of a single workspaceStorage bucket and filtered by "last workspace used in", so they remain visible and continuable from other windows/locations.
4. **Machine-portable history** (from @gidimiz-ST on #313970): either bind history to project identity rather than machine-local storage, or provide an official export/import so the same repo on another PC can pick up prior conversations.

## Related

- Previous attempt: #313970 (closed after upvote window; contains additional discussion)
- Older related reports: #152415, #257143
- Parallel requests in other agent tools, which already track the same need:
- Codex: https://github.com/openai/codex/issues/15347 (assigned)
- Claude Code: https://github.com/anthropics/claude-code/issues/88903
- OpenCode: https://github.com/anomalyco/opencode/issues/44256

Consistent behavior across these tools would remove a recurring pain point for users who work with several agents on the same projects.