feat: make terminal the primary workspace surface

docs/decisions/0008-primary-terminal-3x1-workspace.md

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+# 0008: The default workspace is a primary-terminal 3×1 composition
+
+Status: accepted for the native vertical slice.
+
+Lumbridge defaults to three vertical bands in one column. The top 20% exposes
+pane, agent, tools, context, goal, target, and attention state. The middle 60%
+belongs to the active terminal or work surface. The bottom 20% holds answers,
+choices, chat, approvals, and the local-analyst boundary.
+
+This composition favors the surface where engineers type and read for most of
+the day. It keeps context and decisions visible without making six equally sized
+cards compete for attention. Pane selectors and keyboard shortcuts switch the
+large work surface. Users may later create deliberate splits or dashboard grids,
+but those are workspace choices rather than the default visual hierarchy.
+
+The framework spike retains six updating surfaces even when five are offscreen.
+That preserves the earlier streaming and reducer workload for performance
+comparison. Hidden surfaces do not receive keyboard input merely because they
+continue to update.
+
+Window geometry determines the middle band's terminal rows and columns. The UI
+resizes the terminal engine and PTY through the bounded runtime actor; it does
+not resize only the text view or infer terminal wrapping after the fact.