fix: address final review — AAAI→EMNLP, chs→Video-Tree-TRM, D7 config boundary

- ARCHITECTURE.md: fix target conference AAAI→EMNLP
- subagent-driven-development SKILL.md: fix remaining chs env reference
- .env.example: remove EMBED_BACKEND/EMBED_MODEL (D7: belongs in YAML)
- config/default.yaml: clarify as research experiment config source

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

.env.example

@@ -30,9 +30,7 @@ ASR_API_KEY=gsk-xxx
 # ── MonkeyOCR ──
 MONKEY_OCR_URLS=http://10.77.0.20:7866,http://10.77.0.20:7867
 
-# ── Embedding（远程模式时使用）──
-EMBED_BACKEND=local
-EMBED_MODEL=BAAI/bge-base-zh-v1.5
+# ── Embedding（仅远程模式的敏感凭证；backend/model_name 由科研 YAML 管理）──
 EMBED_API_KEY=
 EMBED_API_URL=