🤖 feat: move Mermaid instructions into built-in diagram skill

[ethanndickson]

Summary

Move Mermaid diagram authoring guidance out of the always-on system PRELUDE and into a new built-in mux-diagram skill that agents load on-demand via agent_skill_read.

Relates to #2334

Background

Every model request currently receives ~200 tokens of Mermaid best-practice instructions in the system prompt, even though the vast majority of requests never produce a diagram. This is a vestige of the pre-skills approach to context injection and wastes tokens on every single request.

Implementation

• New built-in skill src/node/builtinSkills/mux-diagram.md — contains the migrated Mermaid best practices plus a new section on text-based chart alternatives (markdown tables, ASCII bars, indented trees) for cases where full Mermaid rendering is unnecessary.
• Trimmed PRELUDE — the <markdown> block in systemMessage.ts now has a single-line routing hint ("load the built-in mux-diagram skill via agent_skill_read for best practices") instead of the full Mermaid instruction set. Generic markdown guidance (katex, <details>/<summary>) remains.
• Namespaced as mux-diagram — follows the mux-* convention (matching mux-docs) so project/global skills don't accidentally shadow the built-in.
• Test expectations updated — two assertions in agentSkillsService.test.ts that hardcode the built-in skill list now include "mux-diagram".

Risks

Low — the Mermaid instructions are still available to the agent; they're just loaded on-demand instead of injected unconditionally. If diagram quality regresses, the routing hint in PRELUDE should be sufficient to prompt the model to load the skill.

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Generated with mux • Model: anthropic:claude-opus-4-6 • Thinking: xhigh • Cost: $5.66

[chatgpt-codex-connector]

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Codex Review: Didn't find any major issues. 👍

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