docs: add workspaces page explaining project/workspace/session model

Adds a new conceptual Workspaces doc that explains how Paseo organizes work around projects, workspaces, and sessions. Reframed the existing Git worktrees page as the technical deep-dive it always was. Updated glossary with agent session and tab terminology. Reordered docs nav so workspaces and worktrees come before providers.

public-docs/cli.md

@@ -2,7 +2,7 @@
 title: CLI
 description: "Paseo CLI reference: manage agents, daemons, permissions, and worktrees from your terminal."
 nav: CLI
-order: 6
+order: 8
 ---
 
 # CLI