Add gpt-oss-20b architecture support plan

Research and implementation plan for OpenAI's gpt-oss MoE model.
Covers GGUF tensor layout, bias support gaps, SwiGLU-OAI activation,
attention sinks, and alternating SWA. Estimated ~365 LOC across 7 files.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: unamedkr