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.

.claude/skills/astro

@@ -0,0 +1 @@
+../../.agents/skills/astro

.claude/skills/framer-motion

@@ -0,0 +1 @@
+../../.agents/skills/framer-motion

.claude/skills/shadcn

@@ -0,0 +1 @@
+../../.agents/skills/shadcn

.claude/skills/tailwindcss

@@ -0,0 +1 @@
+../../.agents/skills/tailwindcss

.claude/skills/tailwindcss-advanced-layouts

@@ -0,0 +1 @@
+../../.agents/skills/tailwindcss-advanced-layouts

.claude/skills/vercel-react-best-practices

@@ -0,0 +1 @@
+../../.agents/skills/vercel-react-best-practices