fix(security): harden remaining inline onclick handlers against XSS double-context

Extends PR #132 (ultracode handlers) to the rest of the frontend. The same
JS-string-in-HTML-attribute pattern — '${escapeHtml(value)}' — remained in 32
more inline handlers across app.js, panels-ui.js, session-ui.js,
subagent-windows.js, and notification-manager.js. The browser HTML-decodes the
attribute value before parsing the handler source, so escapeHtml's ' reverts
to ' and a quote-bearing id/path/name breaks out of the JS string literal into
executable code.

Switch all to escapeHtml(JSON.stringify(value)): JSON.stringify JS-encodes and
quote-wraps first, then escapeHtml handles the HTML-attribute layer, so the
value round-trips as one inert string argument.

Also fixes two non-escapeHtml variants of the same class:
- panels-ui.js: mux-session `sid` was pre-escaped with escapeHtml() then dropped
  into a single-quoted JS string (selectSession / killMuxSession). Now
  JSON.stringify'd at the source.
- orchestrator-panel.js: phase.id was interpolated raw (no escaping at all) into
  orchestratorSkipPhase / orchestratorRetryPhase. Now escapeHtml(JSON.stringify()).

The most realistic vector here is file paths (panels-ui openLogViewerWindow) —
filenames can legally contain a single quote.

Numeric interpolations (${i+1}, ${index}, ${item.version}) and the
developer-literal ${onclick} in orchestrator-panel are not user data and are
left as-is. Verified: 0 vulnerable patterns remain, all 22 frontend files parse
(check:frontend-syntax + node --check), and a runtime round-trip confirms the
injection that fired under the old pattern is now an inert string argument.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/public/subagent-windows.js

@@ -33,10 +33,10 @@ Object.assign(CodemanApp.prototype, {
       const truncatedName = displayName.length > 25 ? displayName.substring(0, 25) + '…' : displayName;
       const statusClass = agent?.status || 'idle';
       agentItems.push(`
-        <div class="subagent-dropdown-item" onclick="event.stopPropagation(); app.restoreMinimizedSubagent('${escapeHtml(agentId)}', '${escapeHtml(sessionId)}')" title="Click to restore">
+        <div class="subagent-dropdown-item" onclick="event.stopPropagation(); app.restoreMinimizedSubagent(${escapeHtml(JSON.stringify(agentId))}, ${escapeHtml(JSON.stringify(sessionId))})" title="Click to restore">
           <span class="subagent-dropdown-status ${statusClass}"></span>
           <span class="subagent-dropdown-name">${escapeHtml(truncatedName)}</span>
-          <span class="subagent-dropdown-close" onclick="event.stopPropagation(); app.permanentlyCloseMinimizedSubagent('${escapeHtml(agentId)}', '${escapeHtml(sessionId)}')" title="Dismiss">&times;</span>
+          <span class="subagent-dropdown-close" onclick="event.stopPropagation(); app.permanentlyCloseMinimizedSubagent(${escapeHtml(JSON.stringify(agentId))}, ${escapeHtml(JSON.stringify(sessionId))})" title="Dismiss">&times;</span>
         </div>
       `);
     }
@@ -699,7 +699,7 @@ Object.assign(CodemanApp.prototype, {
       parentSessionId && parentSessionName
         ? `<div class="subagent-window-parent" data-parent-session="${parentSessionId}">
           <span class="parent-label">from</span>
-          <span class="parent-name" onclick="app.selectSession('${escapeHtml(parentSessionId)}')">${escapeHtml(parentSessionName)}</span>
+          <span class="parent-name" onclick="app.selectSession(${escapeHtml(JSON.stringify(parentSessionId))})">${escapeHtml(parentSessionName)}</span>
         </div>`
         : '';
 
@@ -720,7 +720,7 @@ Object.assign(CodemanApp.prototype, {
           <span class="status ${agent.status}">${agent.status}</span>
         </div>
         <div class="subagent-window-actions">
-          <button onclick="app.closeSubagentWindow('${escapeHtml(agentId)}')" title="Minimize to tab">─</button>
+          <button onclick="app.closeSubagentWindow(${escapeHtml(JSON.stringify(agentId))})" title="Minimize to tab">─</button>
         </div>
       </div>
       ${parentHeader}