docs: add ADR-004 and DeepSeek runtime spike

[JaylanJerry]

Summary

• Add ADR-004: DeepSeek stays on official ACP as the target boundary, but 0.1.1-rc.2 is No-Go for production migration.
• Add the Official ACP Runtime Closure Spike report.
• Stop claiming DeepSeek inherits the user current persistent model. Bridge does not manage DeepSeek model/provider/reasoning effort; the effective route comes from the running Harness ACP composition, and the current legacy integration uses the upstream composition.

Test plan

• Docs-only change; no production code
• Skim README, PRODUCT_GUIDE, and Skill for leftover inheritance claims