feat: add /gsd:next command for automatic workflow advancement (#927) (#1167)

Adds a zero-friction command that detects the current project state and
automatically invokes the next logical workflow step:

- No phases → discuss first phase
- Phase has no context → discuss
- Phase has context but no plans → plan
- Phase has plans but incomplete → execute
- All plans complete → verify and complete phase
- All phases complete → complete milestone
- Paused → resume work

No arguments needed — reads STATE.md, ROADMAP.md, and phase directories
to determine progression. Designed for multi-project workflows.

Closes #927

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, 39, `expected 39 skill folders, got ${dirs.length}`);
+      assert.strictEqual(dirs.length, 40, `expected 40 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 = 39;
+const EXPECTED_SKILLS = 40;
 const EXPECTED_AGENTS = 16;
 
 function runCopilotInstall(cwd) {