No test coverage — TDD keeps getting deferred, despite being exactly the right fit for agentic development

Huge amounts of code get written, refactored, and rebuilt by agents with zero automated tests guarding key functionality. TDD is unusually well-suited to this codebase's working style — agents iterate until green, and tests are the durable spec of 'still works' across rewrites — but we've procrastinated it. Needs: pick the test environment for this stack, then climb incrementally to full coverage and full passing.