chore(wip): checkpoint P8/C avant chantier Codex inter-agents

Sauvegarde de l'arbre de travail en cours (persistance P8, conversations
C-series, write-portal frontend, médiation d'entrée) avant d'attaquer le
support de la délégation inter-agents pour les profils Codex.

Le round-trip inter-agent question/réponse est couvert sans tokens par
les tests loopback existants (state::mcp_e2e_loopback_tests).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/layout/LayoutGrid.tsx

@@ -27,10 +27,9 @@ import type {
   TerminalHandle,
 } from "@/ports";
 import {
-  MediatedInput,
   ResumeConversationPopup,
   TerminalView,
-  useAgentBusy,
+  useWritePortal,
 } from "@/features/terminals";
 import { useGateways } from "@/app/di";
 import { leaves, normalizeWeights, resizeAdjacent } from "./layout";
@@ -203,10 +202,12 @@ function LeafView({ id, session, agent, conversationId, agentWasRunning, cwd, vm
   const siblingIndex = parentSplit ? (parentSplit.index === 0 ? 1 : 0) : 0;
   const { agent: agentGateway, system } = useGateways();
 
-  // Per-agent busy map (lot F1) feeds the mediated input strip: while the agent
-  // is processing a turn, "Envoyer" is dimmed (but the enqueue stays open) and
-  // "Interrompre" is active. Absent key ⇒ idle.
-  const busyMap = useAgentBusy();
+  // The single write-portal of this cell (ARCHITECTURE §20). It owns the human
+  // line counter, the local delegation FIFO, the handshake (b→e) and the overlay
+  // shown while a delegation is being injected. For a plain (agent-less) cell
+  // `agent` is null: the portal stays inert (no subscription, no overlay) and is
+  // simply not passed to the terminal.
+  const { portal, overlay } = useWritePortal(projectId, agent ?? null);
 
   // Load the project's agents for the dropdown.
   const [agents, setAgents] = useState<Agent[]>([]);
@@ -552,44 +553,66 @@ function LeafView({ id, session, agent, conversationId, agentWasRunning, cwd, vm
         )}
       </div>
       {/* Option 1 (Terminal + MCP): every cell — plain or agent — renders the
-          raw xterm {@link TerminalView}. Agent cells are interactive PTYs; their
-          cross-model delegation happens through MCP tools, not a chat view.
-          Re-key on the agent so the right opener is captured at mount; the
-          persisted session drives re-attach (never a re-spawn) when navigating.
-
-          Lot F2 (cadrage §4.2): in an **agent** cell the terminal is output-only
-          for the human (`agentMode`) and the {@link MediatedInput} strip is
-          mounted **under** it — keystrokes route through IdeA, not the PTY. A
-          **plain** cell keeps the raw-shell behaviour (keystrokes → PTY) and has
-          no input strip. We stack terminal + strip in a column so the strip sits
-          beneath the live output. */}
+          raw xterm {@link TerminalView}. Agent cells are **native terminals**
+          (ARCHITECTURE §20): every human keystroke (Enter included) reaches the
+          PTY. There is no mediated-input strip; cross-model delegation flows
+          through the write-portal (which injects at a clean line boundary) and
+          MCP tools. Re-key on the agent so the right opener is captured at mount;
+          the persisted session drives re-attach (never a re-spawn) when
+          navigating. The agent cell passes its {@link WritePortal} so keystroke
+          counting + injection suspension are wired; a plain cell passes none. */}
       <div
         style={{
-          display: "flex",
-          flexDirection: "column",
+          position: "relative",
           width: "100%",
           height: "100%",
           minHeight: 0,
           minWidth: 0,
         }}
       >
-        <div style={{ flex: "1 1 auto", minHeight: 0, minWidth: 0 }}>
-          <TerminalView
-            key={`${id}-${agentId ?? "plain"}`}
-            cwd={cwd}
-            open={terminalOpener}
-            reattach={reattachOpener}
-            sessionId={session}
-            onSessionId={(sid) => void vm.setSession(id, sid)}
-            agentMode={agentId != null}
-          />
-        </div>
-        {agentId != null && (
-          <MediatedInput
-            projectId={projectId}
-            agentId={agentId}
-            busy={busyMap[agentId] ?? false}
-          />
+        <TerminalView
+          key={`${id}-${agentId ?? "plain"}`}
+          cwd={cwd}
+          open={terminalOpener}
+          reattach={reattachOpener}
+          sessionId={session}
+          onSessionId={(sid) => void vm.setSession(id, sid)}
+          agentMode={agentId != null}
+          portal={agentId != null ? portal : undefined}
+        />
+        {/* Write-portal overlay (ARCHITECTURE §20.3 step b/e): while a delegation
+            is being injected into the agent's PTY, a grey veil with a centred
+            message sits above the terminal. Only ever shown for an agent cell. */}
+        {agentId != null && overlay && (
+          <div
+            data-testid="write-portal-overlay"
+            role="status"
+            aria-live="polite"
+            style={{
+              position: "absolute",
+              inset: 0,
+              zIndex: 4,
+              display: "flex",
+              alignItems: "center",
+              justifyContent: "center",
+              background: "rgba(0, 0, 0, 0.45)",
+              color: "var(--color-content, #e0e0e0)",
+              fontSize: 13,
+              pointerEvents: "none",
+              userSelect: "none",
+            }}
+          >
+            <span
+              style={{
+                background: "var(--color-surface, #1e1e1e)",
+                border: "1px solid var(--color-border, #3a3a3a)",
+                borderRadius: 4,
+                padding: "6px 12px",
+              }}
+            >
+              Un agent est en train de parler…
+            </span>
+          </div>
         )}
       </div>
       {busyNotice && (