Feature Request: Folder-Pinned Sessions

### Describe the feature or problem you'd like to solve

It would be helpful to have the ability to pin a Copilot CLI session to a specific folder/repository, so that when I run gh copilot from that directory again, it automatically resumes the pinned session rather than starting fresh.

### Proposed solution

Current behavior:

- gh copilot starts a new session
- gh copilot --resume shows a list of recent sessions to choose from

Problem with current behavoir:

When working across many different repos it's very easy to either loose the session you want to resume from the recent list, or get confused about which session is for which repo.

Desired behavior:

- Ability to pin/bookmark a session to a specific folder
- When running gh copilot from that folder, automatically resume the pinned session
- Perhaps a command like gh copilot --pin-here or similar, or a specific .md document containing a session reference which copilot could look for

Use case: When working on a specific project over multiple days/sessions, I'd like to maintain context and
conversation history without having to search through recent sessions each time.

### Example prompts or workflows

Ability to jump into a project and immediately continue from where I left off, without either hunting for the right session, or having to explain myself again.

A combined history for changes and their reasons associated with the project, very useful to replay for people coming into the project or teams working on the same codebase.

### Additional context

_No response_