chore: remove cortex_prompt.txt (now in cortex-prompt-harness)

- Remove cortex_prompt.txt from repository root
- Update documentation in core.rs to reflect new architecture
- System prompt is now centralized in cortex-prompt-harness crate

The prompt content is now the canonical source in:
cortex_prompt_harness::prompts::CORTEX_MAIN_PROMPT

Author: echobt

Cargo.lock

@@ -636,8 +636,11 @@ impl Default for CortexPromptBuilder {
 ///
 /// # Usage
 ///
-/// This prompt is loaded via `include_str!` from `cortex_prompt.txt` in the
-/// main cortex-engine, but the canonical version is defined here.
+/// This is the canonical source for the Cortex system prompt. It is used
+/// directly by cortex-engine via `cortex_prompt_harness::prompts::CORTEX_MAIN_PROMPT`.
+///
+/// For dynamic prompt building with custom sections and tools, use
+/// [`CortexPromptBuilder`] instead.
 pub const CORTEX_MAIN_PROMPT: &str = r#"# CORTEX
 
 You are **Cortex**, an autonomous software engineering intelligence.