feat: add new OpenSpec skills for change management and onboarding

- Created `openspec-ff-change` skill for fast-forward artifact creation.
- Introduced `openspec-new-change` skill for structured change creation.
- Developed `openspec-onboard` skill for guided onboarding through OpenSpec workflow.
- Added `openspec-sync-specs` skill for syncing delta specs to main specs.
- Implemented `openspec-verify-change` skill for verifying implementation against change artifacts.
- Updated `.gitignore` to exclude OpenSpec generated files.
- Added `skills-lock.json` to manage skill dependencies.

.agents/skills/tailwindcss/references/layout-aspect-ratio.md

@@ -0,0 +1,39 @@
+---
+name: layout-aspect-ratio
+description: Controlling element aspect ratio for responsive media
+---
+
+# Aspect Ratio
+
+Utilities for controlling the aspect ratio of an element (e.g. video, images).
+
+## Usage
+
+```html
+<!-- 16:9 video container -->
+<div class="aspect-video">
+  <iframe src="..."></iframe>
+</div>
+
+<!-- 4:3 -->
+<div class="aspect-[4/3]">Content</div>
+
+<!-- Square -->
+<div class="aspect-square">1:1</div>
+
+<!-- Auto (intrinsic) -->
+<div class="aspect-auto">Natural ratio</div>
+```
+
+## Key Points
+
+- `aspect-video` - 16:9
+- `aspect-square` - 1:1
+- `aspect-auto` - browser default (intrinsic)
+- `aspect-[4/3]` - arbitrary ratio
+- Useful for responsive video embeds and image containers
+
+<!--
+Source references:
+- https://tailwindcss.com/docs/aspect-ratio
+-->