Document breaking change: dotnet tool install --local creates manifest by default in .NET 10

[Copilot]

This PR documents the breaking change introduced in .NET 10 Preview 7 where dotnet tool install --local now creates a manifest by default when one doesn't exist, instead of failing with a "Cannot find a manifest file" error.

Changes Made

1. New Breaking Change Documentation

Created docs/core/compatibility/sdk/10.0/dotnet-tool-install-local-manifest.md that documents:

• Previous behavior: Command failed with error when no manifest existed
• New behavior: Automatically creates manifest using --create-manifest-if-needed logic by default
• Type: Behavioral breaking change
• Reason: Improves UX by making local tool installation work out-of-the-box
• Recommended action: Users can opt out with --create-manifest-if-needed=false

2. Updated Tool Installation Documentation

Updated docs/core/tools/dotnet-tool-install.md to reflect the new default behavior:

• Modified the Local tools section to clarify that manifests are created automatically starting in .NET 10
• Enhanced the --create-manifest-if-needed option description to explain it's now enabled by default
• Added information about how users can explicitly opt out of the new behavior

Background

This change was implemented by making createManifestIfNeeded enabled by default in the .NET SDK. The manifest creation follows the same rules as the existing --create-manifest-if-needed flag:

1. Look for a directory with a .git subfolder
2. Look for a directory with a .sln or .git file
3. Fall back to current working directory

While this improves the user experience, it's a breaking change since users may have relied on the failure behavior to detect when they needed to create a manifest.

Fixes #48123.

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Internal previews

📄 File	🔗 Preview link
docs/core/compatibility/10.0.md	Breaking changes in .NET 10
docs/core/compatibility/sdk/10.0/dotnet-tool-install-local-manifest.md	dotnet tool install --local creates manifest by default
docs/core/compatibility/toc.yml	docs/core/compatibility/toc
docs/core/tools/dotnet-tool-install.md	dotnet tool install