Move chrome zone content type into chrome-state

The formatter already produced ChromeZoneContent but imported the type
from the TUI shell, so chrome-state depended on the paint module that
consumes it. The type now lives next to formatChromeZones. Paint helpers
stay in the shell. FormattedChromeZones stays a distinct always-populated
shape; the partial ChromeZoneContent still treats omitted fields as
don't-touch.

Author: TheGreatAxios

src/tui/chrome-state.ts

@@ -41,7 +41,6 @@ import {
   type LaneState,
 } from "./agent-progress.js";
 import { AGENTS_PANEL_MAX_VISIBLE, TASKS_PANEL_MAX_VISIBLE } from "./geometry/zones.js";
-import type { ChromeZoneContent } from "./shell.js";
 
 /**
  * How long a finished agent row (done / failed / cancelled / interrupted) stays
@@ -157,6 +156,18 @@ export interface FormattedChromeZones {
   readonly agents: readonly AgentPanelRow[] | null;
 }
 
+/**
+ * Partial chrome snapshot for `setChromeZones`. Omitted fields mean don't-touch
+ * (leave the current zone); `null`/empty means hide. Distinct from
+ * `FormattedChromeZones`, which always names both zones.
+ */
+export interface ChromeZoneContent {
+  /** One row per task-panel line. Null/empty = hide the zone. */
+  readonly task?: readonly TaskPanelRow[] | null;
+  /** One row per agents-panel line. Null/empty = hide the zone. */
+  readonly agents?: readonly AgentPanelRow[] | null;
+}
+
 /**
  * Format structured live state into chrome zone rows for setChromeZones.
  *

src/tui/shell.ts

@@ -6,7 +6,12 @@
  */
 
 import { homedir } from "node:os";
-import { clampBoardRows, type AgentPanelRow, type TaskPanelRow } from "./chrome-state.js";
+import {
+  clampBoardRows,
+  type AgentPanelRow,
+  type ChromeZoneContent,
+  type TaskPanelRow,
+} from "./chrome-state.js";
 
 import {
   BoxRenderable,
@@ -4792,13 +4797,6 @@ export function dispatchPaletteSelection(shell: AppShell, cmd: PaletteCommand):
   });
 }
 
-export interface ChromeZoneContent {
-  /** One row per task-panel line. Null/empty = hide the zone. */
-  readonly task?: readonly TaskPanelRow[] | null;
-  /** One row per agents-panel line. Null/empty = hide the zone. */
-  readonly agents?: readonly AgentPanelRow[] | null;
-}
-
 /** Bracket marker per task status; a trailer row (status null) gets none. */
 function taskStatusMarker(status: TaskPanelRow["status"]): string {
   switch (status) {