Port of buildSubAgentStyleGuideInstruction (orchestrator-sub-agent-compact.ts):
look up the planner-selected guide by name (select_style_guide + the registry),
extract its palette/fonts/radius (extract_style_guide_values), and append a
tier-aware "follow these specs" block to the sub-agent system prompt so the
model uses the selected aesthetic instead of inventing a conflicting palette.
RUST ADAPTATION: TS emits $color-* refs (which it seeds into doc.variables);
Rust does not seed style-guide vars, so refs wouldn't resolve — we emit the
guide's CONCRETE HEX values instead. Same effect, correct for the Rust path.
This also CLOSES the design-system gap the earlier fix left open: when the
style-guide block is injected it REPLACES design-system, so design_system_covered
now also includes style_guide_instruction.is_some() — a named guide no longer
falls back to the generic (and conflicting) design-system skill.
Tests: a named style guide injects "VISUAL STYLE GUIDE" + drops design-system;
the no-style-guide path still drops design-system via style-defaults.
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