fix(session): support symlinked config directories

[fcoury-oai]

Why

A symlinked ~/.config/red can load configuration, but session snapshots fail to save because the snapshot store rejects symlinked path components. This leaves users with dotfile-managed config directories without session recovery data.

What

Resolve the configured directory to its physical path before selecting the sessions root at editor startup. Both snapshot writes and --resume use that path. The snapshot store still rejects symlinks within sessions and its owner directories, as enforced by its directory traversal.

How to Test

1. In a disposable home or XDG config directory, symlink red to a writable config directory, then launch red and exit normally. Confirm sessions/editor-*/latest.json appears in the target directory without a snapshot failure warning.
2. Launch red --resume with the same config path and confirm it loads the saved session.
3. In a separate disposable config directory, make sessions itself a symlink. Confirm Red still rejects that session root.

Targeted test passed: cargo test --bin red session_root_resolves_a_symlinked_config_directory.