Do not automatically install arcade-skills plugins

[lbussell]

Background

The existing Copilot settings were actively harming my work in this repo and other repos. Having 3-4 MCP servers and extra skills enabled by default was bloating the context by a few thousand tokens when they aren't needed for every task. Additionally, the plugin is not disabled when leaving the repo.

Changes

This PR changes the following:

• Removed the setting to automatically install the dotnet-dnceng plugin. The marketplace stays, so the plugin is still easy to install. If you already have the plugin installed, then it doesn't get uninstalled or disabled.
• Added an MOTD/announcement explaining how to easily install the plugin. This pops up when you start copilot in the repo.
• Added dotnet/skills as another known plugin.
• Deleted .claude/settings.json. It was duplicated code that we aren't getting benefit from. In the last month in this repo:
  • Commits made Copilot CLI: 44
  • Commits made by Claude Code: 0
• Removed leftover symlink
• Added Copilot local settings to .gitignore so all of this can be overridden locally if desired.

[Copilot]

Pull request overview

Removes automatic installation/enabling of the dotnet-dnceng Copilot plugin to avoid unnecessary context/tooling overhead by default, while keeping plugin discovery and install guidance in-repo.

Changes:

• Removed the Copilot enabledPlugins default that auto-enables dotnet-dnceng@dotnet-arcade-skills.
• Added a Copilot CLI announcement (MOTD-style) telling users how to install the plugin on demand.
• Added dotnet/skills as an additional known marketplace and updated .gitignore for user-local agent settings; removed duplicated Claude settings file.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
.gitignore	Adds ignore entries for user-local Claude/Copilot CLI settings files.
.github/copilot/settings.json	Removes auto-enabled plugin config; adds known marketplace and an install announcement.
.claude/settings.json	Deletes duplicated Claude settings configuration.

[lbussell]

@MichaelSimons, you are most likely the heaviest user of the dotnet-dnceng plugin in this repo, so that is why I requested your review here.

[lbussell]

/ba-g docs-only change.