feat(sessions): add engine-isolated private contexts #370

Job	Run time
Web app	33s
macOS build and protocol suite	2m 13s
Linux E2E (Docker)	3m 2s
Protocol suite	1m 16s
Agent runtime suite	14s
npm launcher	13s
Static checks	17s
macOS E2E (WKWebView)	-1s
	7m 47s

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