feat: add Turkish language support

[umutuluer]

Summary

• Add getLang() helper to centralize language resolution with fallback to "en"
• Add Turkish translations for all dimension labels and prompts
• Add Turkish translations for orchestrator, single-agent, fixer, and toggle prompts
• Add Turkish translations for all 5 dimension review prompts (code-quality, security, performance, testing, documentation)
• Add Turkish translation for output format templates and command descriptions
• Update README to document "tr" language option

Test plan

• Verify language: "tr" renders Turkish prompts for all agents
• Verify language: "zh" still renders Chinese (no regression)
• Verify language: "en" still renders English (no regression)
• Verify unknown language falls back to "en"
• Verify custom_rules still appear correctly in Turkish single-agent mode