Refactor prompt builder to use Go templates

[mariusvniekerk]

Summary

• The prompt system currently uses string constants with inline prose conditionals (e.g. "If a tag is present...") to handle variable prompt content. As the prompt logic grows more conditional, this becomes fragile.
• Switch to text/template for system prompt rendering so conditional sections (intent-alignment, review type variants, agent-specific instructions) are structural {{if}} blocks rather than natural-language hedges.
• The builder would assemble a data struct describing what's available (commit message present, review type, agent capabilities) and render the appropriate template.

Spawned from #579.