feat: add marketplace-architect skill with Bash scripts and evals

Adds a new skill for creating, managing, and adopting plugins across
Claude Code and GitHub Copilot CLI marketplaces. Includes three Bash
scripts (inventory, gen_manifests, validate), three reference docs
(cross-compat, claude-code, copilot-cli), a test harness with 18
passing tests, and an eval.yaml. Also adds the `marketplace` category
to CATEGORIES.md and commits the plugin marketplace architecture
research doc that informed the skill design.

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

.agents/skills/marketplace-architect/META.md

@@ -0,0 +1,21 @@
+```yaml
+version: "1.0"
+updated: 2026-06-20
+
+when: >
+  Invoked when the user wants to create, manage, or maintain a plugin marketplace for
+  Claude Code and/or GitHub Copilot CLI. Covers four operations: (a) audit and refactor
+  a repository of skills/agents/hooks into a plugin marketplace layout, (b) adopt external
+  plugins/skills/agents from outside sources, (c) update and maintain an existing
+  marketplace.json and plugin manifests, (d) validate existing plugin manifests for naming,
+  structure, and cross-tool compatibility. Also triggered implicitly when the user asks
+  about distributing skills to a team, organizing loose skills into installable units, or
+  setting up cross-tool distribution — even without saying "marketplace".
+
+references:
+  - https://code.claude.com/docs/en/plugins
+  - https://code.claude.com/docs/en/plugin-marketplaces
+  - https://code.claude.com/docs/en/plugins-reference
+  - https://docs.github.com/en/copilot/how-tos/copilot-cli/customize-copilot/plugins-creating
+  - https://docs.github.com/en/copilot/how-tos/copilot-cli/customize-copilot/plugins-marketplace
+```