slice 8C Phase B.1: agentapi disk client (Disks/AssignDisk/EjectDisk/FormatDisk)

ErrFormatRefused surfaces the agent's data-bearing refusal distinctly. Tests:
list, blank format OK, data-bearing refused, eject dependents.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

controller/internal/agentapi/client.go

@@ -174,6 +174,99 @@ func (c *Client) BackupStatus(ctx context.Context) (StatusResponse, error) {
 	return out, nil
 }
 
+// ---- slice 8C: disk management (execution is the agent's) --------------------------------
+
+// DiskInfo mirrors the agent's GET /disks entry.
+type DiskInfo struct {
+	Name          string `json:"name"`
+	Type          string `json:"type"`
+	State         string `json:"state"`
+	BackingDevice string `json:"backing_device"`
+	MountPath     string `json:"mount_path"`
+	Class         string `json:"class"`
+	DataBearing   bool   `json:"data_bearing"`
+	DataReason    string `json:"data_reason"`
+}
+
+// DisksResponse mirrors GET /disks.
+type DisksResponse struct {
+	VMID  int        `json:"vmid"`
+	Disks []DiskInfo `json:"disks"`
+}
+
+// FormatResult mirrors POST /disks/format (the success/refusal payload).
+type FormatResult struct {
+	VMID        int    `json:"vmid"`
+	Device      string `json:"device"`
+	Formatted   bool   `json:"formatted"`
+	DataBearing bool   `json:"data_bearing"`
+	Reason      string `json:"reason"`
+}
+
+// ErrFormatRefused is returned by FormatDisk when the agent refuses a data-bearing format
+// (pending operator authorization — the 8C invariant). The UI surfaces this distinctly.
+var ErrFormatRefused = fmt.Errorf("agentapi: format refused — device is data-bearing (operator authorization required)")
+
+// Disks lists the host drives the agent manages, with a data-bearing flag per drive.
+func (c *Client) Disks(ctx context.Context) (DisksResponse, error) {
+	var out DisksResponse
+	body, err := c.get(ctx, "/disks")
+	if err != nil {
+		return out, err
+	}
+	if err := json.Unmarshal(body, &out); err != nil {
+		return out, fmt.Errorf("agentapi: decode /disks: %w", err)
+	}
+	return out, nil
+}
+
+// AssignDisk attaches a drive (by fs-UUID) as a host mount (benign, self-serve).
+func (c *Client) AssignDisk(ctx context.Context, uuid, where, fstype, options string) error {
+	_, err := c.post(ctx, "/disks/assign", map[string]string{
+		"uuid": uuid, "where": where, "fstype": fstype, "options": options,
+	})
+	return err
+}
+
+// EjectResult mirrors POST /disks/eject (the dependent-guest warning).
+type EjectResult struct {
+	VMID            int    `json:"vmid"`
+	Ejected         string `json:"ejected"`
+	DependentGuests []int  `json:"dependent_guests"`
+}
+
+// EjectDisk safe-unmounts a host mount (data preserved) and returns the dependent guests.
+func (c *Client) EjectDisk(ctx context.Context, where string) (EjectResult, error) {
+	var out EjectResult
+	body, err := c.post(ctx, "/disks/eject", map[string]string{"where": where})
+	if err != nil {
+		return out, err
+	}
+	if err := json.Unmarshal(body, &out); err != nil {
+		return out, fmt.Errorf("agentapi: decode /disks/eject: %w", err)
+	}
+	return out, nil
+}
+
+// FormatDisk asks the agent to format a device. The AGENT inspects the device and decides
+// data-bearing-ness — a data-bearing device is refused (ErrFormatRefused), the controller's claim
+// is irrelevant. Only a device the agent reads as blank is formatted.
+func (c *Client) FormatDisk(ctx context.Context, device, fstype string) (FormatResult, error) {
+	var out FormatResult
+	body, err := c.post(ctx, "/disks/format", map[string]string{"device": device, "fstype": fstype})
+	if err != nil {
+		// A data-bearing refusal comes back as HTTP 403 (the post helper turns it into an error).
+		if strings.Contains(err.Error(), "HTTP 403") {
+			return FormatResult{Device: device, Formatted: false, DataBearing: true}, ErrFormatRefused
+		}
+		return out, err
+	}
+	if err := json.Unmarshal(body, &out); err != nil {
+		return out, fmt.Errorf("agentapi: decode /disks/format: %w", err)
+	}
+	return out, nil
+}
+
 // get issues an authenticated GET and unwraps the {ok,data,error} envelope.
 func (c *Client) get(ctx context.Context, path string) (json.RawMessage, error) {
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, c.baseURL+path, nil)