pi-agent: Implement progressive Target detection

internal/target/target.go

@@ -40,6 +40,22 @@ type Inspection struct {
 	Evidence string
 }
 
+// Detection is one presentation-independent availability and configuration
+// inspection outcome. Err distinguishes a failed check from a truthful
+// unavailable or disabled result.
+type Detection struct {
+	Availability Availability
+	Inspection   Inspection
+	Err          error
+}
+
+// Detector is the optional structured detection seam. Targets which can
+// preserve lower-level check failures implement it; legacy Targets continue to
+// participate through Availability and Inspect.
+type Detector interface {
+	Detect(context.Context, *profile.Profile) Detection
+}
+
 // OperationResult reports the adapter's immediate operation outcome. When Err
 // is non-nil, Inspection is advisory because it may have been captured before a
 // partial write or command failure. The Application explicitly calls Inspect