Export Mermaid project-structure diagram and add arcgispro diagram command

[danmaps]

Motivation

• Provide a machine-readable and human-friendly visualization of ArcGIS Pro project topology (maps, layers, layouts, map frames) to aid dependency tracing and AI-assisted analysis.
• Surface diagram artifacts in the snapshot so diagrams are exported alongside existing JSON/Markdown outputs for use with tools like beautiful-mermaid.

Description

• Add MapFrameInfo and a MapFrames collection to LayoutInfo so layouts capture both frame name and referenced map name (ProExporter/Models.cs).
• Collect layout map frame metadata in the add-in so layout entries include map-frame → map mappings (ProExporter/ContextCollector.cs).
• Generate Mermaid diagram files during serialization: snapshot/project-structure.mmd (Mermaid source) and snapshot/project-structure.md (Mermaid fenced markdown), and include them in the list of exported snapshot files (ProExporter/Serializer.cs).
• Implement a BuildMermaidDiagram routine that emits a flowchart showing Project → Maps → Layers, group layer nesting, layout → map relationships via map frames, and visually highlights layers used in multiple maps (ProExporter/Serializer.cs).
• Add a new CLI command arcgispro diagram that locates the Mermaid source and optionally renders images using beautiful-mermaid with --format and --renderer options (cli/arcgispro_cli/commands/diagram.py) and wire it into the CLI (cli/arcgispro_cli/cli.py).
• Update arcgispro snapshot to report on existence of the generated diagram files and show them in the snapshot summary (cli/arcgispro_cli/commands/snapshot.py).
• Document the new diagram outputs and command in the top-level README.md and cli/README.md.

Testing

• No automated tests were run as part of this change.

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Codex Task