feat: make terminal the primary workspace surface

docs/ARCHITECTURE.md

@@ -69,12 +69,24 @@ The first `lumbridge-runtime` actor now owns one `PtySession` on a dedicated
 thread. Bounded command and event queues serialize input, resize, close, and
 shutdown against ordered raw-byte output. The GPUI slice feeds one actor session
 through the terminal engine while five surfaces retain deterministic comparison
-output. Its current text adapter intentionally renders plain rows and does not
-yet paint cell styles, selection, or the cursor; the next rendering slice owns
-that work. This actor still runs in-process; moving the same framework-neutral
+output. Its adapter groups adjacent cells into native GPUI paint runs and renders
+ANSI/indexed/RGB colors, emphasis, hyperlinks, and cursor shapes. Window geometry
+drives terminal rows and columns and resizes both the engine and PTY. Selection,
+scrollback navigation, mouse reporting, and a lower-level terminal canvas remain.
+This actor still runs in-process; moving the same framework-neutral
 contract behind local authenticated IPC is the next durability step. See
 decisions 0004 and 0005.
 
+## Default workspace composition
+
+The default desktop workspace is a vertical 3×1 composition: the top 20% holds
+pane, agent, tool, context, and goal state; the middle 60% is the active terminal
+or work surface; the bottom 20% is the answer, choice, chat, and approval shelf.
+Six deterministic surfaces still update in the spike so performance comparisons
+remain meaningful, but only the selected surface owns the large work region.
+Users may deliberately split that region later; a dashboard grid is not the
+calm default. See decision 0008.
+
 We should evaluate, not blindly copy, WezTerm, Zellij, RMUX, tmux, and cmux. The
 first spike must compare a reusable terminal crate with a small first-party layer.
 Remaining correctness cases include OSC 8 links, Kitty