feat: audit log in Files panel sidebar

Adds an "Audit Log" section (collapsed by default) at the bottom of the Files
panel showing tool_audit/YYYY-MM-DD.jsonl files for the current user.

- GET /api/audit/files  — lists available dates (newest first, any auth user)
- GET /api/audit/day    — returns entries for one date as JSON (any auth user)
- tool_audit.read_day() — reads a single day's JSONL file chronologically
- Clicking a date renders a read-only table: time / tool / status / args / result
- Status cells are colour-coded (green ok, red error, amber denied)
- Edit/Raw/Preview/Save buttons are hidden in audit view, restored on file switch
- Audit group starts collapsed; expands on click like other file groups

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/static/style.css

@@ -1232,6 +1232,42 @@
         #file-preview.active      { display: block; }
         #file-editor-wrap.hidden  { display: none; }
 
+        /* ── Audit log table ────────────────────────────────────────── */
+        .audit-table {
+            width: 100%;
+            border-collapse: collapse;
+            font-size: 0.78rem;
+            table-layout: fixed;
+        }
+        .audit-table th {
+            text-align: left;
+            padding: 5px 8px;
+            border-bottom: 1px solid var(--border);
+            color: var(--muted);
+            font-weight: 600;
+            white-space: nowrap;
+        }
+        .audit-table td {
+            padding: 5px 8px;
+            border-bottom: 1px solid color-mix(in srgb, var(--border) 50%, transparent);
+            vertical-align: top;
+            overflow: hidden;
+            text-overflow: ellipsis;
+            white-space: nowrap;
+        }
+        .audit-table tr:last-child td { border-bottom: none; }
+        .audit-table tr:hover td { background: var(--surface); }
+        /* Column widths */
+        .at-time   { width: 7em;  color: var(--muted); white-space: nowrap; }
+        .at-tool   { width: 11em; color: var(--accent); font-weight: 500; }
+        .at-status { width: 4.5em; font-weight: 600; }
+        .at-args   { width: 30%; color: var(--muted); }
+        .at-result { color: var(--muted); }
+        .at-status.ok     { color: #4ade80; }
+        .at-status.error  { color: #f87171; }
+        .at-status.denied { color: #fbbf24; }
+        .audit-empty { padding: 24px; color: var(--muted); text-align: center; font-size: 0.9rem; }
+
         /* Talk activity badge on Sessions button */
         #sessions-btn.talk-badge::after {
             content: '●';