fix(store): add missing WithAgentKey context function

Commit a8815a94 introduced a call to store.WithAgentKey() in
agent/loop.go but never added the function to store/context.go,
breaking the build on main.

internal/store/context.go

@@ -29,6 +29,8 @@ const (
 	SharedKGKey contextKey = "goclaw_shared_kg"
 	// ShellDenyGroupsKey holds per-agent shell deny group overrides.
 	ShellDenyGroupsKey contextKey = "goclaw_shell_deny_groups"
+	// AgentKeyKey is the context key for the agent key/name (string identifier, e.g. "default").
+	AgentKeyKey contextKey = "goclaw_agent_key"
 	// TenantIDKey is the context key for the tenant UUID.
 	TenantIDKey contextKey = "goclaw_tenant_id"
 	// CrossTenantKey indicates the caller has cross-tenant access (owner/system admin).
@@ -89,6 +91,19 @@ func AgentTypeFromContext(ctx context.Context) string {
 	return ""
 }
 
+// WithAgentKey returns a new context with the agent key/name (string identifier).
+func WithAgentKey(ctx context.Context, key string) context.Context {
+	return context.WithValue(ctx, AgentKeyKey, key)
+}
+
+// AgentKeyFromContext extracts the agent key from context. Returns "" if not set.
+func AgentKeyFromContext(ctx context.Context) string {
+	if v, ok := ctx.Value(AgentKeyKey).(string); ok {
+		return v
+	}
+	return ""
+}
+
 // WithSenderID returns a new context with the original individual sender ID.
 func WithSenderID(ctx context.Context, id string) context.Context {
 	return context.WithValue(ctx, SenderIDKey, id)