feat: add marketplace.json sync validation and fix Copilot schema

Applies the same shared-field sync logic to the two marketplace.json
files that we already applied to plugin.json: name, owner.name,
description, version, and the full plugin catalog (name, source,
description per entry) must match across .claude-plugin/ and
.github/plugin/ manifests.

Fixes .github/plugin/marketplace.json to use the canonical Copilot
schema (description and version nested under metadata) rather than
a copy of Claude's top-level format. Adds owner field to both files.
Validate.sh handles the structural divergence by extracting values
from either location before comparing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -1,5 +1,6 @@
 {
   "name": "holocron",
+  "owner": { "name": "Your Name", "email": "you@example.com" },
   "description": "AI development skills for Claude Code and GitHub Copilot CLI — factory, design, implement, review, and cross-cutting workflows.",
   "version": "0.1.0",
   "plugins": [