Inbox notification count (● Nmsg) should be per-project, not global

Problem

The status bar shows ● 4msg (inbox notification count) globally across all projects. When switching between different project directories, the same notification count persists — messages from unrelated projects clutter the current workspace.

Expected behavior

The inbox notification count should be scoped to the current project directory, similar to how .claude/settings.json and other project-level config are already per-directory.

Why it matters

When working on multiple projects, global notifications are noise. Users can't tell which messages are relevant to their current context. This defeats the purpose of having per-project workspaces.

Suggested fix

Scope inbox messages by project root (the directory containing .claude/), and only show the count for the active project in the status bar.