feat(conversation): cheap-default DeepSeek + Enter-send + model pill

- default-interactive@1 model: claude-haiku-4-5 → deepseek/deepseek-chat
  (input $0.28/$1.12 per 1M; haiku 대비 ~75% 절감).  fallback 은 haiku 로 swap.
- conversation textarea keydown:
  - Enter → 전송 (IME composition 중이면 무시)
  - Shift+Enter → 줄바꿈
  - Cmd/Ctrl+Enter → 전송 (백워드 호환)
  - Placeholder 안내 갱신.
- conversation top-bar 에 model pill 추가 (#session-model-pill) — 현재 세션의
  활성 model 을 monospace badge 로 표시.  헷갈리던 "어느 모델인가?" 해소.
- style.css 에 .conv-model-pill (회색 pill).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

my-deepagent/docs/schemas/personas/default-interactive@1.yaml

@@ -2,8 +2,8 @@ name: default-interactive
 version: 1
 description: "interactive 모드 만능 어시스턴트. 탐색·수정·실행 모두 지원."
 backend: openrouter
-model: "openrouter:anthropic/claude-haiku-4-5"
-provider_origin: "US/Anthropic"
+model: "openrouter:deepseek/deepseek-chat"
+provider_origin: "CN/DeepSeek"
 capabilities:
   - spec_write
   - code_edit
@@ -42,7 +42,7 @@ allowed_tools:
   - write_todos
   - task
 deepagents_backend: local_shell
-fallback_model: "openrouter:deepseek/deepseek-chat"
+fallback_model: "openrouter:anthropic/claude-haiku-4-5"
 max_cost_per_call_usd: 0.05
 model_params:
   max_tokens: 2048