Restore valid Markdown delimiter runs in the guidance observer #476

🤖 Generated by the Agentic Engineer

Problem

The merged guidance observer rejects valid double-backtick Markdown spans. On main at 59c8266, Use ``gh pr view --json state,mergedAt``. returns UNKNOWN, although the preceding guard accepts this literal command. Character parity cannot distinguish a Markdown delimiter run from shell substitution.

Acceptance criteria

Track matching Markdown delimiter runs so valid single, double and triple code spans, including earlier completed spans on the same line, remain accepted. Preserve UNKNOWN for leading or in-word shell substitutions, escapes and incomplete quoting, and preserve real standalone closing fences. Add actual observer/guard regression coverage and retain all existing checks. Inspected guidance must never execute. Size: S.

Part of #51. Related roadmap: #38.