460 add agent skill flag

[SeanAlexanderHarris]

Summary

• Adds a global --skill flag so an AI agent/workflow invoking the CLI can identify itself, for observability into agentic CLI usage.
• Supersedes the original plan (a dedicated X-Prolific-Agent-Skill header) with a skill/<value> token folded into the existing User-Agent header, alongside the agent/<value> token from Add ai detection to include the agent or model using the cli Closes #459 #462 — avoids relying on a custom header that's likely scrubbed by the backend.
• Wires the composed User-Agent into the three request paths that build their own *http.Request and bypass Client.Execute() (dataset upload, collection export download, batch export download), none of which previously sent any User-Agent at all.

Stacked on #462 (459-add-agent-use-detection) since this builds directly on the agentenv AI-agent detection it adds.

Closes #460

Test plan

• go build ./...
• go test ./... (excluding the pre-existing, unrelated contract_test.TestAPICoverage drift on this branch lineage)
• golangci-lint run ./...
• Manually verified with PROLIFIC_DEBUG=true: --skill cli-command-create study list sends User-Agent: prolific-oss/cli/<version> agent/claude-code skill/cli-command-create

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