ci/docker: bump Primus base image to v26.5

[WangLingxun]

Summary

• upgrade the Primus CI base image and active launch defaults from v26.4 to v26.5
• align README and example documentation with the v26.5 release and MaxText image
• remove the obsolete v26.4 rocSHMEM team-symbol shim and document v26.5 SDK discovery

Test plan

• Build the CI-derived image locally with the production build arguments
• Verify Primus-Turbo, Triton, and rocSHMEM ODC operators import on MI300X
• Run 8-GPU, 3-step Megatron-LM Llama 3 8B E2E training
• Run 8-GPU, 3-step TorchTitan Llama 3.1 8B BF16 E2E training
• Run pre-commit, ShellCheck, YAML, diff, and version-consistency checks