fix(prompt): separate review findings

[wesm]

Summary

• Adds an instruction in review prompt templates (default, dirty, range, security, plus claude-code/codex/gemini variants) telling agents to separate multiple findings with --- on its own line, so findings no longer render as a single block.
• Adds TestGetSystemPrompt_ReviewPromptsRequireFindingSeparators to assert the separator instruction is present across all review prompt variants.
• Refreshes prompt golden files to include the new line.

Fixes #697

[roborev-ci]

roborev: Combined Review (1cce01d)

All review agents found no Medium, High, or Critical issues.

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Synthesized from 3 reviews (agents: codex, gemini | types: default, security)

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[roborev-ci]

roborev: Combined Review (b2a95b4)

Verdict: No Medium, High, or Critical issues found across the reviews.

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Synthesized from 3 reviews (agents: codex, gemini | types: default, security)