feat: publish PSModule process marketplace (#520)

## Summary

- Add the reusable Pester 6.1.0 migration skill and documentation.
- Publish the canonical `psmodule` plugin through the PSModule-owned
`psmodule` marketplace.
- Use the initiative plugin layout `.github/plugin/<plugin-name>/`
without an intermediate `plugins/` directory.
- Name the skill `psmodule-pester-migration` so skill IDs identify their
owning plugin.
- Keep marketplace and plugin metadata at version `1.0.0`.

## Validation

- Marketplace and plugin manifests parse successfully and use
schema-compliant lowercase IDs.
- Repository-relative plugin source resolves to
`.github/plugin/psmodule`.
- Documentation build completed successfully with Zensical.

---------

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

Author: MariusStorhaug

.github/plugin/README.md

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+# PSModule process plugin
+
+This repository hosts the `psmodule` GitHub Copilot CLI marketplace and its
+`psmodule` plugin. The marketplace and plugin are owned and released
+with the PSModule process.
+
+## Install through the marketplace
+
+Register this repository's marketplace and install the plugin:
+
+```console
+copilot plugin marketplace add https://github.com/PSModule/Process-PSModule.git
+copilot plugin install psmodule
+```
+
+## Install directly
+
+Install the plugin directly from this repository:
+
+```console
+copilot plugin install PSModule/Process-PSModule:.github/plugin/psmodule
+```
+
+The plugin provides the
+[`psmodule-pester-migration`](./psmodule/skills/psmodule-pester-migration/SKILL.md)
+skill for migrating every Pester test set in a PSModule repository to Pester
+6.1.0, and the
+[`psmodule-v8-upgrade`](./psmodule/skills/psmodule-v8-upgrade/SKILL.md)
+skill for upgrading Process-PSModule consumer repositories to framework v8.
+It also provides
+[`psmodule-zensical-migration`](./psmodule/skills/psmodule-zensical-migration/SKILL.md)
+for migrating legacy MkDocs sites to the Process-PSModule Zensical design.
+Verify the installation with:
+
+```console
+copilot plugin list
+/skills list
+```
+
+The marketplace catalog is in [`marketplace.json`](./marketplace.json), and
+plugin metadata is in
+[`plugin.json`](./psmodule/plugin.json).

.github/plugin/marketplace.json

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+{
+  "$schema": "https://agent-plugins.org/schemas/1.0.0/marketplace.schema.json",
+  "name": "psmodule",
+  "owner": {
+    "name": "PSModule"
+  },
+  "metadata": {
+    "description": "Agent plugins for the PSModule repository process.",
+    "version": "1.0.0"
+  },
+  "plugins": [
+    {
+      "name": "psmodule",
+      "description": "PSModule process guidance and reusable skills for PowerShell module repositories.",
+      "version": "1.0.0",
+      "source": ".github/plugin/psmodule"
+    }
+  ]
+}

.github/plugin/psmodule/plugin.json

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+{
+  "$schema": "https://agent-plugins.org/schemas/1.0.0/plugin.schema.json",
+  "name": "psmodule",
+  "description": "PSModule process guidance and reusable skills for PowerShell module repositories.",
+  "version": "1.0.0",
+  "author": {
+    "name": "PSModule",
+    "url": "https://github.com/PSModule"
+  },
+  "homepage": "https://psmodule.io/docs/",
+  "repository": "https://github.com/PSModule/Process-PSModule",
+  "license": "MIT",
+  "keywords": [
+    "powershell",
+    "psmodule",
+    "pester",
+    "testing",
+    "github-actions"
+  ],
+  "skills": "skills/"
+}