Simplify INSTALL.md — let convey onboarding handle config
Removed the manual config-writing section entirely — convey's
first-run wizard handles password, identity, and API key setup.
Prerequisites simplified to just git + uv (Makefile handles the
rest). Observer install defers to observer repo INSTALL.md.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>