feat: community-requested commands — /gsd:review, /gsd:plant-seed, /gsd:pr-branch

Three commands reimplemented from positively-received community PRs:

1. /gsd:review (#925) — Cross-AI peer review
   Invoke external AI CLIs (Gemini, Claude, Codex) to independently
   review phase plans. Produces REVIEWS.md with per-reviewer feedback
   and consensus summary. Feed back into planning via --reviews flag.
   Multiple users praised the adversarial review concept.

2. /gsd:plant-seed (#456) — Forward-looking idea capture
   Capture ideas with trigger conditions that auto-surface during
   /gsd:new-milestone. Seeds preserve WHY, WHEN to surface, and
   breadcrumbs to related code. Better than deferred items because
   triggers are checked, not forgotten.

3. /gsd:pr-branch (#470) — Clean PR branches
   Create a branch for pull requests by filtering out .planning/
   commits. Classifies commits as code-only, planning-only, or mixed,
   then cherry-picks only code changes. Reviewers see clean diffs.

All three are standalone command+workflow additions with no core code
changes. Help workflow updated. Test skill counts updated.

797/797 tests pass.

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, 43, `expected 43 skill folders, got ${dirs.length}`);
+      assert.strictEqual(dirs.length, 46, `expected 46 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 = 43;
+const EXPECTED_SKILLS = 46;
 const EXPECTED_AGENTS = 16;
 
 function runCopilotInstall(cwd) {