docs: add ACP and internal systems implementation guides

- add ACP user and developer docs covering setup, lifecycle, callbacks,
  permissions, tool rendering, and runtime behavior
- add developer guides for agent loop, provider runtime resolution,
  prompt assembly, context caching/compression, gateway internals,
  session storage, tools runtime, trajectories, and cron internals
- refresh architecture, quickstart, installation, CLI reference, and
  environments docs to link the new implementation pages and ACP support

website/sidebars.ts

@@ -42,6 +42,8 @@ const sidebars: SidebarsConfig = {
             'user-guide/messaging/discord',
             'user-guide/messaging/slack',
             'user-guide/messaging/whatsapp',
+            'user-guide/messaging/signal',
+            'user-guide/messaging/email',
             'user-guide/messaging/homeassistant',
           ],
         },
@@ -81,6 +83,7 @@ const sidebars: SidebarsConfig = {
           type: 'category',
           label: 'Integrations',
           items: [
+            'user-guide/features/acp',
             'user-guide/features/mcp',
             'user-guide/features/honcho',
             'user-guide/features/provider-routing',
@@ -101,6 +104,16 @@ const sidebars: SidebarsConfig = {
       label: 'Developer Guide',
       items: [
         'developer-guide/architecture',
+        'developer-guide/agent-loop',
+        'developer-guide/provider-runtime',
+        'developer-guide/prompt-assembly',
+        'developer-guide/context-compression-and-caching',
+        'developer-guide/gateway-internals',
+        'developer-guide/session-storage',
+        'developer-guide/tools-runtime',
+        'developer-guide/acp-internals',
+        'developer-guide/trajectory-format',
+        'developer-guide/cron-internals',
         'developer-guide/environments',
         'developer-guide/adding-tools',
         'developer-guide/creating-skills',