Clarify agent issue body formatting rules

[davidsneighbour]

Plain Markdown issue bodies created by agents can accidentally render labels as large headings, and Markdown-heavy text passed through gh issue create --body can be corrupted by shell interpretation.

Update .agents/instructions/issue-handling.instructions.md so future issue creation:

• avoids Markdown headings in issue bodies unless a structured template is explicitly requested,
• prefers plain labels or bullet lists for agent-created issues,
• uses gh issue create --body-file for Markdown-heavy issue bodies, command examples, code spans, backticks, or multi-paragraph text.