Classify worked items as session artifacts

[Benjamin Christopher Simmonds (benibenj)]

Summary

• classify issues and pull requests a session works on as artifacts even when it did not create them
• align the artifact tool schema, description, system instruction, and validation error
• add regression coverage for the model guidance

Testing

• Not run (per request)

[Copilot]

Copilot review overview

🟡 Changes recommended

The regression test does not verify the classification’s essential subject and creation condition.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review tier: Balanced
Findings: 1

New issues introduced by this change (1)

Severity	Finding
	src/​vs/​platform/​agentHost/​test/​node/​artifactServerTools.test.ts — This checks only the generic fragment “is an artifact even if”, so the regression still passes if…

What changed in this PR

Updates artifact guidance so worked-on issues and pull requests count as session artifacts, even when created elsewhere.

Changes:

• Aligns tool descriptions, schema guidance, and validation messaging.
• Adds regression coverage for model-facing guidance.

File	Description
artifactServerTools.ts	Updates artifact classification guidance.
sessionArtifactCollection.ts	Aligns validation error wording.
artifactServerTools.test.ts	Tests guidance across model-facing surfaces.

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