FR: Recreate missing Git worktree when resuming a session

## Problem

A session can reference a Git worktree that no longer exists. Selecting that session fails before agent starts:

```text
Error: failed to change directory to /path/to/repo/.worktrees/example

Caused by:
No such file or directory (os error 2)
```

This commonly happens after local worktree cleanup while session history remains available.

## Reproduction

Tested with `fr 2.10.0` and Pi:

1. Create Git repository and branch.
2. Create worktree for branch.
3. Start Pi inside worktree and create session.
4. Exit Pi.
5. Remove worktree while keeping branch.
6. Run `fr`.
7. Select session.

`fr` exits with status 1 and does not recreate worktree.

I reproduced this twice in an isolated temporary repository. As a control, I recreated worktree and selected same session. `fr` then resumed Pi from correct directory with correct session ID.

## Requested behavior

When saved directory is missing and appears to be Git worktree, offer to recreate it before resuming session.

For example:

```text
Session worktree is missing:
/path/to/repo/.worktrees/example

Recreate it from branch `example`? [y/N]
```

Then run equivalent of:

```text
git worktree add /path/to/repo/.worktrees/example example
```

Automatic recovery could be limited to unambiguous cases:

- Parent Git repository can be found.
- Matching local branch exists.
- Target path does not exist.
- Branch is not checked out in another worktree.

If recovery is ambiguous, current failure can remain with more actionable context.

## Current implementation

Resume handoff calls `set_current_dir()` before agent execution. Missing directories fail immediately. There is no Git worktree recovery path.