feat: full Copilot SDK feature parity — custom agents, hooks, image input, skills, steering, session persistence

Backend (sidecar):
- Extended ProtocolModels with DTOs for custom agents, hooks, skills,
  infinite sessions, session lifecycle, and 9 new event types
- Added CopilotManagedSessionIds for stable SDK session ID mapping
- Added CopilotSessionManager/ICopilotSessionManager for session lifecycle
- Added CopilotSessionHooks for hook registration
- Added CopilotMessageOptionsMetadata for mid-turn steering
- Extended CopilotAgentBundle to wire custom agents, hooks, skills,
  infinite sessions, and stable session IDs
- Extended CopilotTurnExecutionState to project 13 new SDK event types
- Widened ITurnWorkflowRunner callback to accept SidecarEventDto
- Added list/delete/disconnect session commands to SidecarProtocolHost
- Added AryxCopilotAgentMessageOptionsTests (14 new tests, 142 total)

Frontend (renderer + main + shared):
- Added ChatMessageAttachment type and helpers (attachment.ts)
- Extended sidecar contracts with MessageMode, 3 new command types,
  9 new event types, and agent/session config DTOs
- Extended SessionEventRecord with 6 new event kinds and ~20 fields
- Added PatternAgentCopilotConfig to pattern domain
- Added attachments support to ChatMessageRecord
- Updated sidecar client with session lifecycle methods and
  turn-scoped event routing via onTurnScopedEvent callback
- Updated main process: handleTurnScopedEvent(), deleteSession(),
  steering bypass for mid-turn messages, attachment passthrough
- Added deleteSession IPC handler and preload binding
- Added TurnEventLog state tracker with format/apply/prune helpers
- ChatPane: always-enabled composer, steering indicator, attachment
  picker with preview, image thumbnails in message history,
  context-usage bar, amber steer mode for send button
- ActivityPanel: turn events section with sub-agent, hook, skill,
  and compaction event rendering
- Sidebar: delete session action in context menu
- App.tsx: wired sessionUsage, turnEventLogs, and deleteSession

Documentation:
- AGENTS.md: added glob safety rule for node_modules
- README.md: added steering, image input, and richer observability
- ARCHITECTURE.md: added turn-scoped events, steering, and attachments
- Website: added steering and image input feature cards, updated
  live visibility and session cards

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

sidecar/tests/Aryx.AgentHost.Tests/CopilotWorkflowRunnerTests.cs

@@ -649,7 +649,7 @@ public sealed class CopilotWorkflowRunnerTests
         Assert.Equal("agent-handoff-ux", observedAgent.AgentId);
         Assert.Equal("UX Specialist", observedAgent.AgentName);
         Assert.Equal("agent-handoff-ux", state.ActiveAgent?.AgentId);
-        AgentActivityEventDto activity = Assert.Single(state.DrainPendingActivityEvents());
+        AgentActivityEventDto activity = Assert.Single(state.DrainPendingEvents().OfType<AgentActivityEventDto>());
         Assert.Equal("thinking", activity.ActivityType);
         Assert.Equal("agent-handoff-ux", activity.AgentId);
     }
@@ -675,7 +675,7 @@ public sealed class CopilotWorkflowRunnerTests
 
         Assert.Equal("agent-handoff-ux", state.ActiveAgent?.AgentId);
         Assert.Equal("UX Specialist", state.ActiveAgent?.AgentName);
-        AgentActivityEventDto activity = Assert.Single(state.DrainPendingActivityEvents());
+        AgentActivityEventDto activity = Assert.Single(state.DrainPendingEvents().OfType<AgentActivityEventDto>());
         Assert.Equal("thinking", activity.ActivityType);
         Assert.Equal("agent-handoff-ux", activity.AgentId);
     }
@@ -699,7 +699,7 @@ public sealed class CopilotWorkflowRunnerTests
                   "timestamp": "2026-03-27T00:00:00Z"
                 }
                 """));
-        _ = state.DrainPendingActivityEvents();
+        _ = state.DrainPendingEvents();
         RequestInfoEvent requestInfo = CreateRequestInfoEvent(
             CreateHandoffTarget("agent-handoff-ux", "UX Specialist"));
         List<AgentActivityEventDto> activities = [];
@@ -715,9 +715,9 @@ public sealed class CopilotWorkflowRunnerTests
                 Array.Empty<ChatMessage>(),
                 state,
                 (Func<TurnDeltaEventDto, Task>)(_ => Task.CompletedTask),
-                (Func<AgentActivityEventDto, Task>)(activity =>
+                (Func<SidecarEventDto, Task>)(sidecarEvent =>
                 {
-                    activities.Add(activity);
+                    activities.Add(Assert.IsType<AgentActivityEventDto>(sidecarEvent));
                     return Task.CompletedTask;
                 }),
             ])!;