feat(FN-3539): remove duplicate todos entry and update navigation docs

Merged branch removes the duplicate desktop "Todos" nav entry from the Header, syncs the `allowParallelExecution` runtime toggle into `AgentDetailView`, and updates the corresponding docs (agents.md, settings-reference.md, todo-view.md, dashboard-guide.md) to reflect the navigation change. Tests wer

Fusion-Task-Id: FN-3539

packages/core/src/db.ts

@@ -165,6 +165,7 @@ CREATE TABLE IF NOT EXISTS tasks (
   paused INTEGER DEFAULT 0,
   baseBranch TEXT,
   branch TEXT,
+  executionStartBranch TEXT,
   baseCommitSha TEXT,
   modelPresetId TEXT,
   modelProvider TEXT,
@@ -1145,6 +1146,10 @@ export class Database {
   private migrate(): void {
     const version = this.getSchemaVersion() || 1;
 
+    if (this.hasTable("tasks")) {
+      this.addColumnIfMissing("tasks", "executionStartBranch", "TEXT");
+    }
+
     if (version >= SCHEMA_VERSION) return;
 
     if (version < 2) {