cmd/sgai: bootstrap jj in embedded IDE sessions

[ucirello]

Summary

• prepare workspace-scoped code-server state on the host so embedded IDE sessions can manage their own shell startup files, cache, and bootstrap hooks
• download, verify, cache, and install a workspace-local jj binary during IDE startup while keeping the container PATH on the stock system defaults
• surface bootstrap failures back through IDE startup and cover the bootstrap script, PATH rewrite, and early container-stop paths with Go regression tests

Test Plan

• make absorb-sgai
• go test ./cmd/sgai -run 'TestBuildDockerRunArgs|TestPrepareIDEWorkspaceState|TestResolveIDEStartError|TestWaitForStartedTargetReturnsBootstrapErrorImmediatelyWhenContainerStops'