Remove mermaid-cli dependency

[josh]

Summary

• drop mermaid-cli from the plugins package inputs and snacks extra packages
• relax the snacks health checks to ignore missing Mermaid CLI warnings

Testing

• nix flake check --accept-flake-config --show-trace --print-build-logs --keep-going

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Codex Task

[Copilot]

Pull Request Overview

This PR removes the mermaid-cli dependency from the lazy-nvim-nix package to reduce dependencies and simplify the build. Since mermaid diagram rendering is an optional feature, the health checks are updated to ignore the expected warnings about missing Mermaid CLI.

• Removed mermaid-cli from package inputs and snacks.nvim extra packages
• Updated health check ignore lists to suppress mermaid-related warnings

Reviewed Changes

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

File	Description
plugins.nix	Removed mermaid-cli from function parameters and snacks.nvim extraPackages list
pkgs/tests/snacks-nvim-checkhealth.nix	Added ignore patterns for mmdc tool not found error and mermaid diagram warning
pkgs/LazyVim.nix	Added ignore patterns for mmdc tool not found error and mermaid diagram warning in snacks health check

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