feat(FN-2869): merge fusion/fn-2869

- docs(FN-2869): pluggable notification providers reference + architecture
- chore: changeset for pluggable notification providers
- ui(merge): clarify mergeConflictStrategy option labels — both "smart" and "prefer-main" run the full smart cascade; differ only in the final fallback (-X theirs vs -X ours)
- TaskDetailModal: keep Node Routing summary block removed

Conflicts in executor.ts, core/index.ts, and RoutingTab.* resolved by
keeping HEAD — fn-2869 branched from a stale main and was dragging in
older versions of files that have since been updated by sibling tasks
(including the FN-2887 rebase fix on executor.ts itself).

packages/dashboard/app/components/SettingsModal.tsx

@@ -2998,13 +2998,21 @@ export function SettingsModal({
                   setForm((f) => ({ ...f, mergeConflictStrategy: e.target.value as "smart" | "ai-only" | "prefer-main" | "abort" }))
                 }
               >
-                <option value="smart">Smart — AI, then auto-resolve, then prefer task branch (default)</option>
-                <option value="ai-only">AI only — never silently pick a side</option>
-                <option value="prefer-main">Prefer main — fall back to keeping main's version</option>
-                <option value="abort">Abort — fail merge if AI can't resolve (manual review)</option>
+                <option value="smart">Smart, prefer task on fallback — AI → auto-resolve → -X theirs (default, original behavior)</option>
+                <option value="prefer-main">Smart, prefer main on fallback — AI → auto-resolve → -X ours (protects just-merged sibling work)</option>
+                <option value="ai-only">AI only — AI → auto-resolve → AI retry; never silently pick a side</option>
+                <option value="abort">Abort — one AI attempt; require manual resolution if it fails</option>
               </select>
               <small>
-                What to do on the final merge attempt when AI and auto-resolve haven't fully resolved the conflict. <strong>Smart</strong> matches the historical behavior — the task branch wins on hard conflicts, which is fast but can resurrect code an earlier sibling task deleted. <strong>Prefer main</strong> sides with main instead, protecting just-merged sibling work. <strong>AI only</strong> retries the AI agent rather than silently choosing a side. <strong>Abort</strong> requires manual resolution for any conflict the AI can't handle.
+                Every option starts with the same first two attempts: an AI agent resolves the conflict, then auto-resolve handles lock/generated/trivial files. They differ only in the <em>final fallback</em>:
+                {" "}
+                <strong>Smart, prefer task</strong> uses <code>-X theirs</code> so the task branch wins — fast, but can resurrect code an earlier sibling task deleted (the FN-2887 class of regression).
+                {" "}
+                <strong>Smart, prefer main</strong> uses <code>-X ours</code> so main wins instead — same speed, protects just-merged sibling work.
+                {" "}
+                <strong>AI only</strong> retries the AI agent rather than auto-picking a side.
+                {" "}
+                <strong>Abort</strong> stops after the first AI attempt and waits for a human.
               </small>
             </div>
             <div className="form-group">

packages/dashboard/app/components/TaskDetailModal.tsx

@@ -1702,6 +1702,31 @@ export function TaskDetailModal({
             </div>
           )}
           <MergeDetails task={task} />
+          <div className="detail-section">
+            <h4>Node Routing</h4>
+            <dl className="detail-source-grid">
+              <div>
+                <dt>Task Override</dt>
+                <dd>{task.nodeId ?? <span className="detail-source-empty">(none)</span>}</dd>
+              </div>
+              <div>
+                <dt>Effective Node</dt>
+                <dd>{(task as Task & { effectiveNodeId?: string }).effectiveNodeId ?? "local execution"}</dd>
+              </div>
+              <div>
+                <dt>Routing Source</dt>
+                <dd>{(task as Task & { effectiveNodeSource?: string }).effectiveNodeSource ?? "local"}</dd>
+              </div>
+              <div>
+                <dt>Unavailable Node Policy</dt>
+                <dd>{(settings as Settings & { unavailableNodePolicy?: string } | undefined)?.unavailableNodePolicy ?? "block"}</dd>
+              </div>
+              <div>
+                <dt>Blocking Reason</dt>
+                <dd>{((task as Task & { blockedReason?: string; statusReason?: string }).blockedReason || (task as Task & { statusReason?: string }).statusReason) ?? <span className="detail-source-empty">(not blocked)</span>}</dd>
+              </div>
+            </dl>
+          </div>
           {task.sourceIssue && (
             <div className="detail-section detail-source-section">
               <h4>Source Issue</h4>