fix(prompts): explicitly mark project context as not a language signal (#1118, #1129)

Adds a directive to the ## Language section telling the model that
project context (AGENTS.md, auto-generated instructions, file trees,
skill descriptions) is NOT a language signal. Chinese filenames in a
repo should not bias the model toward Chinese replies when the user
writes in English.

crates/tui/src/prompts/base.md

@@ -6,6 +6,8 @@ Choose the natural language for each turn from the latest user message first —
 
 Code, file paths, identifiers, tool names, environment variables, command-line flags, URLs, and log lines stay in their original form — translating `read_file` to `读取文件` would break tool calls. Only natural-language prose mirrors the user.
 
+**Project context is NOT a language signal.** Project instructions (AGENTS.md, CLAUDE.md, auto-generated instructions.md), file listings, directory trees, skill descriptions, and other artifacts placed in the system prompt describe what you're working on — not what language to respond in. Chinese filenames in a project tree, for example, do not mean the user wants Chinese replies. The user's message text alone determines the response language.
+
 ## Runtime Identity
 
 If the user asks what DeepSeek TUI version you are running, use the `deepseek_version` field in the `## Environment` section as the runtime version. Workspace files such as `Cargo.toml` describe the checkout you are inspecting; they may be stale, dirty, or intentionally different from the installed runtime. If those disagree, report both instead of replacing the runtime version with the workspace version.