fix(security): wire appendSecureFile at the four real log-append sites

file-permissions.ts carries a 24-line rationale for why POSIX mode bits are
insufficient on Windows and implements appendSecureFile (0600 at create,
Windows ACL on first write only) — but its single caller was the dead
logAttempt, while the four REAL page-content log writers (console/network/
dialog logs in server.ts, the command audit log) used raw fs.appendFileSync
with no mode. Page-content-derived logs now get owner-only permissions from
birth on every platform.

Verified before wiring: mode applies atomically at create via appendFileSync
{mode}, and the ACL pass runs only on first write — no per-append subprocess
cost on the hot console-log path.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

browse/src/audit.ts

@@ -13,7 +13,7 @@
  * All writes are best-effort — audit failures never cause command failures.
  */
 
-import * as fs from 'fs';
+import { appendSecureFile } from './file-permissions';
 
 export interface AuditEntry {
   ts: string;
@@ -62,7 +62,7 @@ export function writeAuditEntry(entry: AuditEntry): void {
     if (entry.aliasOf) record.aliasOf = entry.aliasOf;
     if (truncatedError) record.error = truncatedError;
 
-    fs.appendFileSync(auditPath, JSON.stringify(record) + '\n');
+    appendSecureFile(auditPath, JSON.stringify(record) + '\n');
   } catch {
     // Audit write failures are silent — never block command execution
   }

browse/src/server.ts

@@ -26,7 +26,7 @@ import {
 } from './content-security';
 import { getStatus as getSecurityStatus } from './security';
 import { isSidecarAvailable, scanWithSidecar } from './security-sidecar-client';
-import { writeSecureFile, mkdirSecure } from './file-permissions';
+import { writeSecureFile, mkdirSecure, appendSecureFile } from './file-permissions';
 import { handleSnapshot, SNAPSHOT_FLAGS } from './snapshot';
 import {
   initRegistry, validateToken as validateScopedToken, checkScope, checkDomain,
@@ -586,7 +586,7 @@ async function flushBuffers() {
       const lines = entries.map(e =>
         `[${new Date(e.timestamp).toISOString()}] [${e.level}] ${e.text}`
       ).join('\n') + '\n';
-      fs.appendFileSync(CONSOLE_LOG_PATH, lines);
+      appendSecureFile(CONSOLE_LOG_PATH, lines);
       lastConsoleFlushed = consoleBuffer.totalAdded;
     }
 
@@ -597,7 +597,7 @@ async function flushBuffers() {
       const lines = entries.map(e =>
         `[${new Date(e.timestamp).toISOString()}] ${e.method} ${e.url} → ${e.status || 'pending'} (${e.duration || '?'}ms, ${e.size || '?'}B)`
       ).join('\n') + '\n';
-      fs.appendFileSync(NETWORK_LOG_PATH, lines);
+      appendSecureFile(NETWORK_LOG_PATH, lines);
       lastNetworkFlushed = networkBuffer.totalAdded;
     }
 
@@ -608,7 +608,7 @@ async function flushBuffers() {
       const lines = entries.map(e =>
         `[${new Date(e.timestamp).toISOString()}] [${e.type}] "${e.message}" → ${e.action}${e.response ? ` "${e.response}"` : ''}`
       ).join('\n') + '\n';
-      fs.appendFileSync(DIALOG_LOG_PATH, lines);
+      appendSecureFile(DIALOG_LOG_PATH, lines);
       lastDialogFlushed = dialogBuffer.totalAdded;
     }
   } catch (err: any) {