fix(minimax-docx): support .NET 8 project restore

[heliusjing]

What

• Restore and build MiniMaxAIDocx.Cli.csproj explicitly in the Bash and PowerShell setup paths.
• Make env_check.sh print the real restore/build error and an exact retry command instead of hiding all output.
• Use the explicit project path for the verification run and quote displayed paths.

Why

On a clean checkout with .NET SDK 8.0.125 / MSBuild 17.8, restoring the scripts/dotnet directory selects MiniMaxAIDocx.slnx and fails with:

MSB4068: The element <Solution> is unrecognized, or not supported in this context.

The existing environment check suppresses that error and reports a generic NuGet failure even when nuget.org, TLS, and the required packages are available. Restoring the CLI .csproj directly restores both referenced projects and works on the declared .NET 8 minimum.

Related to #24. This is intentionally a smaller alternative scoped only to the .slnx / .NET 8 compatibility bug and actionable diagnostics; it does not add offline-feed behavior.

Verification

• bash -n skills/minimax-docx/scripts/env_check.sh skills/minimax-docx/scripts/setup.sh
• Clean bash skills/minimax-docx/scripts/env_check.sh on .NET SDK 8.0.125: Status: READY
• bash skills/minimax-docx/scripts/setup.sh --minimal --skip-verify
• bash skills/minimax-docx/scripts/setup.sh --minimal: builds successfully and creates the verification DOCX
• python3 .claude/skills/pr-review/scripts/validate_skills.py --path skills/minimax-docx: pass

The repository-wide validator still fails on the pre-existing skills/minimax-multimodal-toolkit name mismatch (mmx-cli vs. directory name); minimax-docx passes.

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