Update Copilot docs review instructions: emdash and context rules, fit 4k limit

[maartenba]

Updates .github/instructions/docs.instructions.md used by Copilot code review.

• Copilot code review only reads the first 4,000 characters of an instruction file. The file was 4,070, so trailing rules were ignored. It is now ~3,040.
• Adds a rule to avoid emdashes, preferring split sentences over ;.
• Adds a Context section: include enough context (spec references, one sentence of "why"), as short as possible and as long as needed.
• Deduplicates and condenses existing rules without dropping any.