Batch 5: Lint apps/help/*_system_prompt.md (runtime LLM system prompts)

[lisa-tarbo]

Part of #27.

code_generate_system_prompt.md and filter_system_prompt.md are the most directly agent-critical files in the repo — literal text injected into an LLM at runtime by apps/help/agents/, not documentation. Currently hidden by the blanket apps/ ignore line. 2 files, 4 issues.

Steps

• Add <!-- markdownlint-disable MD041 --> to the top of both files — they're raw prompt payloads, not doc pages, so forcing a # Title first line would be wrong
• Try un-ignoring just these two files via a .markdownlintignore negation pattern (!apps/help/*_system_prompt.md). Caveat: gitignore-style negation doesn't always re-include a file whose parent directory was excluded by a broader pattern (apps/) — test it. If it doesn't work, defer these two files to batch 6 instead (fixed together with the apps/ ignore-line removal)
• Fix 3× MD040 (unlabeled fenced code block) in code_generate_system_prompt.md (lines 29, 121, 144)
• Verify: npx markdownlint-cli2 --config .markdownlint.yaml "apps/help/*.md" is clean