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>

tests/copilot-install.test.cjs

@@ -625,7 +625,7 @@ describe('copyCommandsAsCopilotSkills', () => {
       // Count gsd-* directories — should be 31
       const dirs = fs.readdirSync(tempDir, { withFileTypes: true })
         .filter(e => e.isDirectory() && e.name.startsWith('gsd-'));
-      assert.strictEqual(dirs.length, 50, `expected 50 skill folders, got ${dirs.length}`);
+      assert.strictEqual(dirs.length, 53, `expected 53 skill folders, got ${dirs.length}`);
     } finally {
       fs.rmSync(tempDir, { recursive: true });
     }
@@ -1119,7 +1119,7 @@ const { execFileSync } = require('child_process');
 const crypto = require('crypto');
 
 const INSTALL_PATH = path.join(__dirname, '..', 'bin', 'install.js');
-const EXPECTED_SKILLS = 50;
+const EXPECTED_SKILLS = 53;
 const EXPECTED_AGENTS = 17;
 
 function runCopilotInstall(cwd) {