feat(viz-platform): add design-gate command for binary pass/fail validation

Add /design-gate command that provides binary pass/fail validation for
design system compliance. This command is used by projman orchestrator
during sprint execution to gate issue completion for Domain/Viz issues.

Features:
- Binary PASS/FAIL output for automation
- Checks only FAIL-level violations (invalid props, missing components)
- Integrates with projman sprint execution workflow
- Lightweight alternative to /design-review

Closes #360

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/viz-platform/commands/design-gate.md

@@ -0,0 +1,93 @@
+---
+description: Design system compliance gate (pass/fail) for sprint execution
+arguments:
+  - name: path
+    description: File or directory to validate
+    required: true
+---
+
+# /design-gate
+
+Binary pass/fail validation for design system compliance. Used by projman orchestrator during sprint execution to gate issue completion.
+
+## Usage
+
+```
+/design-gate <path>
+```
+
+**Examples:**
+```
+/design-gate ./app/pages/dashboard.py
+/design-gate ./app/components/
+```
+
+## What It Does
+
+1. **Activates** the `design-reviewer` agent in gate mode
+2. **Loads** the `skills/design-system-audit.md` skill
+3. **Scans** target path for DMC usage
+4. **Checks only for FAIL-level violations:**
+   - Invalid component props
+   - Non-existent components
+   - Missing required props
+   - Deprecated components
+5. **Returns binary result:**
+   - `PASS` - No blocking violations found
+   - `FAIL` - One or more blocking violations
+
+## Output
+
+### On PASS
+```
+DESIGN GATE: PASS
+No blocking design system violations found.
+```
+
+### On FAIL
+```
+DESIGN GATE: FAIL
+
+Blocking Issues (2):
+1. app/pages/home.py:45 - Invalid prop 'onclick' on dmc.Button
+   Fix: Use 'n_clicks' for click handling
+
+2. app/components/nav.py:12 - Component 'dmc.Navbar' not found
+   Fix: Use 'dmc.AppShell.Navbar' (DMC v0.14+)
+
+Run /design-review for full audit report.
+```
+
+## Integration with projman
+
+This command is automatically invoked by the projman orchestrator when:
+
+1. An issue has the `Domain/Viz` label
+2. The orchestrator is about to mark the issue as complete
+3. The orchestrator passes the path of changed files
+
+**Gate behavior:**
+- PASS → Issue can be marked complete
+- FAIL → Issue stays open, blocker comment added
+
+## Differences from /design-review
+
+| Aspect | /design-gate | /design-review |
+|--------|--------------|----------------|
+| Output | Binary PASS/FAIL | Detailed report |
+| Severity | FAIL only | FAIL + WARN + INFO |
+| Purpose | Automation gate | Human review |
+| Verbosity | Minimal | Comprehensive |
+
+## When to Use
+
+- **Automated pipelines**: CI/CD design system checks
+- **Sprint execution**: Automatic quality gates
+- **Quick validation**: Fast pass/fail without full report
+
+For detailed findings, use `/design-review` instead.
+
+## Requirements
+
+- viz-platform MCP server must be running
+- Target path must exist