Add contextual framing to establish Simplex purpose upfront

Added opening statement that establishes what Simplex is designed for:
"Simplex is a specification language for describing work that autonomous
AI agents will perform to develop software systems and artifacts. It
translates human requirements into precise, unambiguous specifications
that AI agents can interpret and execute."

This context-setting helps the model understand:
- WHO the audience is (autonomous AI agents, not humans)
- WHAT problem Simplex solves (requirements → executable specs)
- WHY precision matters (AI agents need unambiguous input)

By framing the purpose before diving into technical rules, the model
can better understand the rationale behind strict requirements like
complete example coverage, structural precision, and DATA type
definitions.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: brannn

web/cmd/server/static/js/simplex-spec.js

@@ -287,7 +287,9 @@ CONSTRAINT: backward_compatibility
   existing v1 API clients must work without modification`;
 
 // System prompt for generation (used with temperature 0.3, max_tokens 2048)
-const BASE_SYSTEM_PROMPT = `You are an expert Simplex specification generator. You must generate specifications that strictly follow the Simplex v0.5 specification.
+const BASE_SYSTEM_PROMPT = `Simplex is a specification language for describing work that autonomous AI agents will perform to develop software systems and artifacts. It translates human requirements into precise, unambiguous specifications that AI agents can interpret and execute.
+
+You are an expert Simplex specification generator. You must generate specifications that strictly follow the Simplex v0.5 specification.
 
 ${SIMPLEX_FULL_SPEC}