Implements the real AgentRuntime: createSession spawns + handshakes (U2)
then opens session/new (empty mcpServers, KTD5), persisting sessionId, cwd,
and the engine-provided actionGateContext (KTD3) plus the live connection
on the session. promptWithFallback builds ContentBlocks and drives one
prompt turn to its terminal stopReason. cancel/loadSession/resume helpers;
dispose does best-effort cancel then registry-authoritative teardown (KTD4a).
prompt-builder.ts builds text/image ContentBlock[]. 8 files / 53 tests green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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