v0.318.0: a part is offered for formatting only while nothing on its disk is mounted (agent 0.158.1 formattable)

The agent's root mkfs wrapper (security review of agent 0.158.0) refuses any mounted node and trusts no sibling claim; the
wizard now matches it. A free filesystem part of an in-use disk stays offered for use. Red-proved.

Claude-Session: https://claude.ai/code/session_012qRErfCoiTkvDK9N5XHbzb

controller/internal/agentapi/client.go

@@ -500,6 +500,9 @@ type CandidatePartition struct {
 	Mountable bool   `json:"mountable"`
 	DurableID string `json:"durable_id,omitempty"` // uuid:<fs-uuid> — the attach key
 	MountedAt string `json:"mounted_at,omitempty"`
+	// Formattable (agent ≥ 0.158.1): false for every part of a disk with a mounted node — the root mkfs wrapper refuses
+	// those and trusts no „it is Felhom's own" claim. Only formattable parts are offered for formatting.
+	Formattable bool `json:"formattable"`
 }
 
 // Partition states (agent ≥ 0.158.0).