Add --mermaid flag for diagram output in depends command

[richlander]

Summary

Add --mermaid flag to the depends command, enabling Mermaid diagram output for type hierarchies, library references, and package dependencies.

Two modes

Flags	Output	Use case
--mermaid	Standalone mermaid (graph TD)	Pipe to mmdc, embed in tooling
--markdown --mermaid	Mermaid fenced blocks inside markdown	Render in GitHub, VS Code, etc.

Examples

# Standalone mermaid
dotnet-inspect depends 'INumber<TSelf>' --mermaid

# Embedded in markdown
dotnet-inspect depends 'INumber<TSelf>' --markdown --mermaid

# Package dependencies as mermaid
dotnet-inspect depends --package System.Text.Json --mermaid

Changes

• OutputFormat.cs — Added Mermaid enum value, IsEmbeddedMermaid() helper, env var support (DOTNET_INSPECT_FORMAT=mermaid)
• SharedOptions.cs — Added --mermaid CLI option, wired into format resolution
• DependsOptions.cs — Added MermaidOutput and EmbeddedMermaid properties
• SearchCommandDefinitions.cs — Wired flags into all 3 depends subcommands (type/library/package)
• DependsCommand.cs — Added WriteMermaidTree() and WriteEmbeddedMermaidTree() helpers, mermaid branches in all 3 Execute methods
• Directory.Packages.props — Updated Markout to 0.12.0 (adds MermaidFormatter)

Dependencies

Requires Markout 0.12.0 which adds MermaidFormatter.

[richlander]

FYI: @StephenCleary