rebrand + purge orphaned frontend: Chat Switchboard → Switchboard Core

Branding: bulk rename across 50+ files (Go, JS, CSS, HTML, YAML, JSON,
shell scripts). Fix Helm chart description and git repo URLs.
Fix test helper taglines.

Admin surface: strip 14 gutted tabs (providers, models, personas,
roles, knowledge, memory, tasks, health, routing, capabilities,
channels, dashboard, usage, stats). Collapse to 4 categories:
People, Workflows, System, Monitoring.

Settings surface: strip 11 gutted tabs (models, personas, providers,
roles, knowledge, memory, usage, workflows, tasks, data, gitkeys).
Keep: general, appearance, profile, teams, connections, notifications.

Team-admin surface: strip 5 gutted tabs (personas, providers,
knowledge, tasks, usage). Keep: members, groups, connections,
workflows, settings, activity.

Components: delete chat-pane/ (13 files, 1938 lines) and
notes-pane/ (10 files, 2381 lines).

main.go: remove stale Health.Prune maintenance goroutine.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/js/sw/surfaces/admin/settings.js

@@ -195,7 +195,7 @@ export default function SettingsSection() {
                 <label class="toggle-label"><input type="checkbox" checked=${cfg.footer_enabled} onChange=${e => set('footer_enabled', e.target.checked)} /><span class="toggle-track"></span><span>Show footer bar</span></label>
                 ${cfg.footer_enabled && html`
                     <div style="margin-top:8px;">
-                        <div class="form-group"><label>Text</label><input value=${cfg.footer_text} onInput=${e => set('footer_text', e.target.value)} placeholder="Powered by Chat Switchboard" /></div>
+                        <div class="form-group"><label>Text</label><input value=${cfg.footer_text} onInput=${e => set('footer_text', e.target.value)} placeholder="Powered by Switchboard Core" /></div>
                     </div>
                 `}
             </div>