docs(agents): add build, test, and architecture guidance

Document the non-obvious facts future sessions would otherwise miss: bun is required for the TUI build, single-test invocation and the custom tsx harness, the test:unit registration requirement, the deliberate e2e skip, generated files, the two-host server/tui split, the excisable Deals slice, and that opencode plugin has no add subcommand. Fix the release skill path to .opencode/skills/release.