Fix .NET SDK prerequisites in CONTRIBUTING.md

[Copilot]

CONTRIBUTING.md incorrectly stated that just is required and omitted Node.js dependencies needed by .NET tests.

Changes

• Mark just as optional (it's a convenience wrapper around direct commands)
• Add Node.js requirement to .NET SDK section (test harness uses TypeScript/tsx)
• Document npm install steps for ./nodejs and ./test/harness directories

The .NET test suite depends on:

• Copilot CLI from nodejs/node_modules/@github/copilot/ (referenced in ClientTests.cs, E2ETestContext.cs)
• TypeScript test harness in test/harness/ using tsx (started by CapiProxy.cs)

Both are installed via npm ci in their respective directories.

Original prompt

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This section details on the original issue you should resolve

<issue_title>CONTRIBUTING.md needs some tweaks</issue_title>
<issue_description>For the .NET client:

• You don't need just
• You need typescript and tpx
• You need to run npm install in the ./nodejs and ./test/harness directories</issue_description>

<agent_instructions>Check that those suggestions make sense and update the relevant document</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes CONTRIBUTING.md needs some tweaks #59

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[Copilot]

Pull request overview

This PR updates the CONTRIBUTING.md documentation to correct prerequisites for the .NET SDK development. The changes clarify that just is optional and document the Node.js dependencies required by the .NET test suite.

Changes:

• Marks the just command runner as optional throughout the document
• Adds Node.js v18+ as a prerequisite for .NET SDK development
• Documents npm installation steps for nodejs and test/harness directories

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[Copilot]

These commands won't work correctly when run sequentially. After the first command, you'll be in the nodejs directory, so the second cd test/harness will fail. Each command should either return to the repository root first, or use separate command blocks. Consider changing to:

cd nodejs && npm ci
cd ../test/harness && npm ci

or

(cd nodejs && npm ci)
(cd test/harness && npm ci)

Suggested change

	cd nodejs && npm ci
	cd test/harness && npm ci
	(cd nodejs && npm ci)
	(cd test/harness && npm ci)