Add contribution CTA to all listing pages (#850)

* Add contribution CTA to all listing/search pages

Add a reusable ContributeCTA component that encourages visitors to
contribute their own resources. The CTA appears at the bottom of every
resource listing page (agents, instructions, skills, hooks, workflows,
plugins, tools) with:

- Contextual text that adapts to the resource type
- 'Contribute yours' primary button linking to CONTRIBUTING.md
- 'Request a resource' outline button linking to new issue creation
- Gradient top bar matching existing card design patterns
- Responsive layout that stacks on mobile

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add Contribute button to page header on all listing pages

Place an outlined Contribute button in each page-header section so it's
immediately visible without scrolling. Uses accent purple border that
fills on hover with a glow effect. Stacks below description on mobile
via a 600px breakpoint.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add screenshots for contribution CTA PR

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* didn't mean to commit them

* Extract PageHeader component to deduplicate header markup

Address PR review feedback: the header Contribute link (URL, SVG icon,
classes) was duplicated across all 7 listing pages. Extract into a
reusable PageHeader.astro component that accepts title and description
props, with a slot for rich HTML descriptions (used by workflows page).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website/src/pages/workflows.astro

@@ -1,16 +1,15 @@
 ---
 import BaseLayout from '../layouts/BaseLayout.astro';
 import Modal from '../components/Modal.astro';
+import ContributeCTA from '../components/ContributeCTA.astro';
+import PageHeader from '../components/PageHeader.astro';
 ---
 
 <BaseLayout title="Workflows" description="AI-powered repository automations that run coding agents in GitHub Actions" activeNav="workflows">
   <main id="main-content">
-    <div class="page-header">
-      <div class="container">
-        <h1>⚡ Agentic Workflows</h1>
-        <p>AI-powered repository automations that run coding agents in <a href="https://gh.io/gh-aw" target="_blank" rel="noopener">GitHub Actions</a></p>
-      </div>
-    </div>
+    <PageHeader title="⚡ Agentic Workflows" description="">
+      AI-powered repository automations that run coding agents in <a href="https://gh.io/gh-aw" target="_blank" rel="noopener">GitHub Actions</a>
+    </PageHeader>
 
     <div class="page-content">
       <div class="container">
@@ -38,6 +37,7 @@ import Modal from '../components/Modal.astro';
         <div class="resource-list" id="resource-list" role="list">
           <div class="loading" aria-live="polite">Loading workflows...</div>
         </div>
+        <ContributeCTA resourceType="workflows" />
       </div>
     </div>
   </main>