feat: bootstrap holocron marketplace with hello-world plugin

Adds marketplace scaffolding (.claude-plugin/marketplace.json,
.github/plugin/marketplace.json) and a hello-world proof-of-concept
plugin to validate the format before migrating real skills.

Also fixes inventory.sh to include .agents/ in the hidden-dir exception
list so audits correctly surface the 16 existing skills.

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

plugins/hello-world/.claude-plugin/plugin.json

@@ -0,0 +1,4 @@
+{
+  "name": "hello-world",
+  "description": "A minimal example plugin to validate marketplace scaffolding."
+}

plugins/hello-world/README.md

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+# hello-world
+
+Minimal proof-of-concept plugin for the `holocron` marketplace. Install it to confirm the marketplace scaffolding works, then replace it with real plugins.
+
+## Install via marketplace
+
+### Claude Code
+```bash
+claude plugin marketplace add <owner>/ai-development
+claude plugin install hello-world@holocron
+```
+
+### GitHub Copilot CLI
+```bash
+copilot plugin marketplace add <owner>/ai-development
+copilot plugin install hello-world
+```
+
+## Local dev install
+
+### Claude Code
+```bash
+claude --plugin-dir ./plugins/hello-world
+```
+
+### GitHub Copilot CLI
+```bash
+copilot plugin install ./plugins/hello-world
+```
+
+## Verify
+
+After install, open a new session and type `/hello-world` or "test plugin install".

plugins/hello-world/plugin.json

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+{
+  "name": "hello-world",
+  "description": "A minimal example plugin to validate marketplace scaffolding."
+}

plugins/hello-world/skills/hello-world/SKILL.md

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+---
+name: hello-world
+description: Prove the holocron marketplace plugin is installed and working. Use when the user says "/hello-world", "test plugin install", or "is the holocron marketplace working". Do NOT use for any real task.
+metadata:
+  category: cross-cutting
+---
+
+Respond with exactly:
+
+> Hello from the **holocron** marketplace! The `hello-world` plugin is installed and working.
+> Run `claude plugin list` to see all installed plugins.
+
+Nothing else.