feat(FN-1375): add null-as-delete pattern for settings with Reset button

- Send null instead of undefined when clearing token cap to explicitly delete setting
- Handle null values as delete operations in updateSettings (for clearing keys)
- Add Reset button in Settings modal to clear token cap with one click
- Update placeholder and help text for token cap input
- Fix TypeScript cast error for config.settings

.fusion/memory.md

@@ -27,6 +27,7 @@
 - Browser directory pickers (`webkitdirectory`) cannot provide a server filesystem path; for dashboard import flows, parse selected `AGENTS.md` files client-side and send `{ agents }` payloads instead of trying to submit a directory `source` path.
 - For conditionally rendered mobile inputs in dashboard components, prefer React `autoFocus` on the input over effect+`setTimeout` focus logic keyed to open-state booleans; mount timing is more reliable and simpler.
 - Checkout leasing is explicit: use `checkoutTask`/`releaseTask` (or `/api/tasks/:id/checkout` + `/release`) for ownership, treat 409 conflicts as non-retryable contention, and let `HeartbeatMonitor.executeHeartbeat()` only validate `checkedOutBy` (never auto-acquire leases).
+- The null-as-delete pattern for settings: In `TaskStore.updateSettings()`, `null` values in the settings patch are treated as "delete this key from settings" (since `JSON.stringify` drops `undefined` keys). This allows the frontend to explicitly clear a setting by sending `null`. The key is deleted from both `config.settings` and `projectPatch` before merging, so cleared settings fall back to `DEFAULT_SETTINGS`.
 
 ## Color Theme System
 

packages/core/src/store.ts

@@ -595,6 +595,17 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
 
     return this.withConfigLock(async () => {
       const config = await this.readConfig();
+
+      // Handle null values as "delete this key from settings"
+      // This allows the frontend to explicitly clear a setting by sending null
+      // (since JSON.stringify drops undefined keys, we use null as a sentinel)
+      for (const key of Object.keys(projectPatch)) {
+        if ((projectPatch as Record<string, unknown>)[key] === null) {
+          delete (config.settings as unknown as Record<string, unknown>)[key];
+          delete (projectPatch as Record<string, unknown>)[key];
+        }
+      }
+
       const globalSettings = await this.globalSettingsStore.getSettings();
       const previousMerged: Settings = { ...DEFAULT_SETTINGS, ...globalSettings, ...config.settings } as Settings;
       const updatedProjectSettings = { ...config.settings, ...projectPatch };

packages/dashboard/app/components/SettingsModal.tsx

@@ -746,17 +746,30 @@ export function SettingsModal({
 
             <div className="form-group">
               <label htmlFor="tokenCap">Token Cap</label>
-              <input
-                id="tokenCap"
-                type="number"
-                placeholder="100000"
-                value={(form as any).tokenCap ?? ""}
-                onChange={(e) => {
-                  const val = e.target.value;
-                  setForm((f) => ({ ...f, tokenCap: val ? parseInt(val, 10) : undefined } as any));
-                }}
-              />
-              <small>Automatically compact context when approaching this token count. Leave empty to use default behavior (compact only on overflow errors).</small>
+              <div style={{ display: "flex", gap: "0.5rem", alignItems: "center" }}>
+                <input
+                  id="tokenCap"
+                  type="number"
+                  placeholder="No cap"
+                  value={(form as any).tokenCap ?? ""}
+                  onChange={(e) => {
+                    const val = e.target.value;
+                    setForm((f) => ({ ...f, tokenCap: val ? parseInt(val, 10) : null } as any));
+                  }}
+                />
+                {(form as any).tokenCap != null && (
+                  <button
+                    type="button"
+                    className="btn btn-ghost btn-sm"
+                    title="Reset to default (no cap)"
+                    onClick={() => setForm((f) => ({ ...f, tokenCap: null } as any))}
+                    style={{ whiteSpace: "nowrap" }}
+                  >
+                    Reset
+                  </button>
+                )}
+              </div>
+              <small>Automatically compact context when approaching this token count. Leave empty for no cap (compact only on overflow errors). Set a number to proactively compact when reaching this token count.</small>
             </div>
 
             {/* --- Planning & Validation --- */}