Add /graph G5: box-drawing DAG rendering for /graph show

/graph show now renders the dependency graph as box-drawing text via a
deterministic Sugiyama-lite pipeline in session/graph_render.rs:
longest-path layering, dagre-style dummy pass-throughs so every drawn
edge spans exactly one layer gap (long edges route as vertical lanes
through intermediate bands), one-pass barycenter ordering, and greedy
bus-lane allocation in the connector gutters with box-drawing-aware
glyph merging (a bus crossing a pass-through renders ┼). Node boxes
carry a status glyph (✓ ▶ ○ · ✗ ⊘) and a clamped title; a legend line
closes the view. Only Blocks edges are drawn — DiscoveredFrom is audit
metadata whose origin is always terminal, so drawing it would double
edges without scheduling meaning.

The output rides ordinary scrollback (the pager already scrolls it), so
no new wire variant or pager view was needed. Honest ceiling: a graph
wider than the 120-column budget — or an empty/pre-planning graph —
degrades to the indented status tree, because box art wrapped by the
terminal is worse than no art; a cycle reaching the renderer (upstream
validation bypassed) refuses to render rather than looping.

Tests: six-node structural snapshot (layering order, fan-in bands,
arrowheads, corners, legend, no trailing whitespace, width bound),
determinism across runs, too-wide fallback, dummy-lane routing through
intermediate bands, DiscoveredFrom exclusion, title clamping, empty
graph; plus a handle_prompt e2e driving /graph show end to end (no-graph
degrade + seeded chain rendering). kigi-shell 4949 lib tests green;
workspace clippy clean.

crates/codegen/kigi-shell/src/session/acp_session_impl/slash_exec.rs

@@ -819,6 +819,22 @@ impl SessionActor {
                 self.send_slash_command_output(&msg).await;
                 ok_end_turn(0, None)
             }
+            BuiltinAction::GraphShow => {
+                // Box-drawing DAG; wider than the budget (or no graph)
+                // degrades to the indented status tree — wrapped box art
+                // is worse than no art.
+                const SHOW_WIDTH_BUDGET: usize = 120;
+                let rendered =
+                    self.graph_tracker.lock().snapshot().and_then(|s| {
+                        crate::session::graph_render::render_dag(s, SHOW_WIDTH_BUDGET)
+                    });
+                let msg = match rendered {
+                    Some(dag) => dag,
+                    None => self.graph_status_message().await,
+                };
+                self.send_slash_command_output(&msg).await;
+                ok_end_turn(0, None)
+            }
             BuiltinAction::GraphPause => {
                 use crate::session::goal_tracker::{GoalPauseReason, GoalStatus};
                 let (msg, changed) = {