feat: /gsd:fast command for trivial inline tasks (#609)

New lightweight command for tasks too small to justify planning overhead:
typo fixes, config changes, forgotten commits, simple additions.

- Runs inline in current context (no subagent spawn)
- No PLAN.md or SUMMARY.md generated
- Scope guard: redirects to /gsd:quick if task needs >3 file edits
- Atomic commit with conventional commit format
- Logs to STATE.md quick tasks table if present

Complements /gsd:quick (which spawns planner+executor) for cases where
the planning overhead exceeds the actual work.

Closes #609

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, 42, `expected 42 skill folders, got ${dirs.length}`);
+      assert.strictEqual(dirs.length, 43, `expected 43 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 = 42;
+const EXPECTED_SKILLS = 43;
 const EXPECTED_AGENTS = 16;
 
 function runCopilotInstall(cwd) {