Write the TUI behavior spec and retire finished migration docs

docs/TUI.md is a normative, code-verified spec for the OpenTUI shell:
layout, overlays, selectors, the palette, the prompt box, and scroll/mouse
behavior. It replaces the interaction-contract and layout-constitution
docs (harvested into it) and the readiness/signoff/cutover/freeze/spike
docs, which recorded a migration that has already landed.

References to the deleted docs are updated to point at docs/TUI.md.

Author: TheGreatAxios

AGENTS.md

@@ -70,6 +70,7 @@ Interchange is the standard library for this repo, consumed as published `@intx/
 ## Reference
 
 - `docs/ARCHITECTURE.md` — reactor loop, events, directors, workflows, plugin chain, permission system
+- `docs/TUI.md` — terminal UI behavior spec: layout, overlays, selectors, palette, prompt box, scrolling
 - `docs/IMPLEMENTATION.md` — runtime, dependencies, config resolution, settings precedence, CLI flags, state persistence, eval harness
 - `docs/PRODUCT.md` — what we're building and why
 - `docs/HOOKS.md` — lifecycle hooks

docs/PRODUCT.md

@@ -36,12 +36,9 @@ $ corbits "Add JWT auth to the API"
 
 A full-screen terminal interface: a pinned header (session title and workflow progress), a scrollable event log, modals for permission prompts and operator questions, and a chat input for follow-up turns.
 
-**Layout and interaction contracts** (OpenTUI is the shipping shell):
-
-- Layout constitution: `docs/tui-layout-constitution.md` (chrome budget, geometry ownership, kill list)
-- Interaction contract: `docs/tui-interaction-contract.md` (queue / steer / interrupt, keys, palette)
-- Migration cutover: `docs/tui-migration-cutover.md`
-- Epic plan: `docs/plans/tui-layout-scroll-platform.md` · product brief: `briefs/tui-rebuild-opentui.md`
+**Behavior spec** (OpenTUI is the shipping shell): `docs/TUI.md` — layout,
+chrome budget, overlays, selectors, palette, prompt box, and scroll/mouse
+behavior.
 
 ### Exec mode (non-TUI product path)