feat: add Copilot connection diagnostics

Add backend support for Copilot connection and account-status UX without introducing raw provider credential management.

This change extends sidecar capabilities with explicit connection diagnostics, adds a refreshable capabilities IPC path, classifies common Copilot CLI failure modes, and includes a frontend handover document describing the new payload and expected UI states.

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

src/shared/contracts/sidecar.ts

@@ -6,6 +6,20 @@ export interface SidecarModeCapability {
   reason?: string;
 }
 
+export type SidecarConnectionStatus =
+  | 'ready'
+  | 'copilot-cli-missing'
+  | 'copilot-auth-required'
+  | 'copilot-error';
+
+export interface SidecarConnectionDiagnostics {
+  status: SidecarConnectionStatus;
+  summary: string;
+  detail?: string;
+  copilotCliPath?: string;
+  checkedAt: string;
+}
+
 export interface SidecarModelCapability {
   id: string;
   name: string;
@@ -17,6 +31,7 @@ export interface SidecarCapabilities {
   runtime: 'dotnet-maf';
   modes: Record<PatternDefinition['mode'], SidecarModeCapability>;
   models: SidecarModelCapability[];
+  connection: SidecarConnectionDiagnostics;
 }
 
 export interface DescribeCapabilitiesCommand {