Adds a session_directory extension event that fires before session manager
creation, allowing extensions to customize the session directory path based
on cwd, git branch, or other context.
- Extensions can return a custom sessionDir in the event handler
- CLI --session-dir flag takes precedence over extension-provided paths
- If multiple extensions return a sessionDir, the last one wins
- Enables implementing branch-based sessions as an extension instead of core feature
This provides the extension point needed to implement git branch-based
session directories without adding the complexity to core.
Closes#1729
7df89066d9