Remove directory structure from system prompt

Remove the getDirectoryStructure() function and its embedding in the
system prompt. This was walking up to 500 files and embedding them as a
static tree in every API request, adding ~3,500-5,000 tokens per call.

The LLM already has tools (listFiles, Glob, Grep) to discover files on
demand, making the embedded tree redundant. Claude Code does not include
directory structure in its system prompt for the same reason.

This also improves prompt cache hit rates since the system prompt no
longer varies by project directory contents.

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Co-Authored-By: Continue <noreply@continue.dev>