feat: add multi-project workspace commands (#1241)

Three new commands for managing isolated GSD workspaces:
- /gsd:new-workspace — create workspace with repo worktrees/clones
- /gsd:list-workspaces — scan ~/gsd-workspaces/ for active workspaces
- /gsd:remove-workspace — clean up workspace and git worktrees

Supports both multi-repo orchestration (subset of repos from a parent
directory) and feature branch isolation (worktree of current repo with
independent .planning/).

Includes init functions, command routing, workflows, 24 tests, and
user documentation.

Closes #1241

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/USER-GUIDE.md

@@ -625,6 +625,31 @@ claude --dangerously-skip-permissions
 /gsd:remove-phase 7         # Descope phase 7 and renumber
 ```
 
+### Multi-Project Workspaces
+
+Work on multiple repos or features in parallel with isolated GSD state.
+
+```bash
+# Create a workspace with repos from your monorepo
+/gsd:new-workspace --name feature-b --repos hr-ui,ZeymoAPI
+
+# Feature branch isolation — worktree of current repo with its own .planning/
+/gsd:new-workspace --name feature-b --repos .
+
+# Then cd into the workspace and initialize GSD
+cd ~/gsd-workspaces/feature-b
+/gsd:new-project
+
+# List and manage workspaces
+/gsd:list-workspaces
+/gsd:remove-workspace feature-b
+```
+
+Each workspace gets:
+- Its own `.planning/` directory (fully independent from source repos)
+- Git worktrees (default) or clones of specified repos
+- A `WORKSPACE.md` manifest tracking member repos
+
 ---
 
 ## Troubleshooting