Agent tab, evidence metrics, and recruiter dashboard focus

What changed

• The Agent surface became a top-level recruiter workspace with history, a full-screen copilot layout, and @-file mentions in assessment chat.
• Public evidence metrics were tightened to use schema-correct definitions for accounts, matches, classroom players, problems, and languages.
• Recruiting dashboard tables, settings, filters, and changelog cards were restyled into denser work surfaces without changing the product palette.