vault, digicert: migrate Token / APIKey to *secret.Ref (Bundle I Phase 3)

Closes Top-10 fix #2 of the 2026-05-03 issuer-coverage audit (see
cowork/issuer-coverage-audit-2026-05-03/RESULTS.md). Pre-fix,
vault.Config.Token and digicert.Config.APIKey were plain string
fields. Practical impact:

  1. GET /api/v1/issuers responses marshalled the credential into
     the JSON body. An acquirer's procurement engineer running
     'curl /api/v1/issuers | jq' saw the token / API key in plain
     text on screen.
  2. DEBUG-level HTTP request logging printed the credential
     header verbatim.
  3. A heap dump of the running server contained the credential
     as readable bytes for the lifetime of the process.

Bundle I from the 2026-05-01 audit closed this for AWSACMPCA,
EJBCA, GlobalSign, Sectigo (Phase 1+2). Vault and DigiCert were
left out. This commit ports the same migration onto them.

Mechanics:
  - Config.Token / Config.APIKey type changed from 'string' to
    '*secret.Ref'. UnmarshalJSON of a JSON string populates the
    Ref via NewRefFromString — operator config files are
    unchanged.
  - Every header-write call site routed through Ref.Use, with the
    byte buffer zeroed after the callback returns. Vault: 3 sites
    (IssueCertificate, RevokeCertificate, GetCACertPEM). DigiCert:
    5 sites (ValidateConfig, IssueCertificate, RevokeCertificate,
    pollOrderOnce, downloadCertificate).
  - ValidateConfig nil-checks switch from 'cfg.Token == ""' to
    'cfg.Token.IsEmpty()' (mirrors Sectigo's existing pattern).
  - Tests migrated: every Config{Token:"..."} →
    Config{Token: secret.NewRefFromString("...")}. The
    'json.Marshal(config) → ValidateConfig(rawConfig)' round-trip
    pattern in DigiCert's ValidateConfig_Success test is now
    broken by the redact-on-marshal contract — switched that one
    to construct the rawConfig as a JSON literal (mirrors
    Sectigo's existing test pattern).
  - Two new tests pin the redact-on-marshal contract:
      - TestVault_Config_TokenMarshalsAsRedacted (vault_redact_test.go)
      - TestDigiCert_Config_APIKeyMarshalsAsRedacted (digicert_redact_test.go)
    Both assert the marshaled JSON contains '"[redacted]"' and
    does NOT contain the plaintext bytes.

Operator-visible: GET /api/v1/issuers responses for type=vault
and type=digicert now show the credential as '[redacted]'.
Existing config files keep working — the Ref unmarshal accepts
strings.

CHANGELOG note: certctl/CHANGELOG.md is intentionally not
hand-edited; release notes are auto-generated from commit
messages between consecutive tags. This commit's message body is
the release-note artifact.

Verified locally:
  - gofmt clean across the repo.
  - go vet ./... clean across the repo.
  - go test -race -count=1 -short
    ./internal/connector/issuer/vault/...
    ./internal/connector/issuer/digicert/...
    ./internal/secret/...  green.

Audit reference: cowork/issuer-coverage-audit-2026-05-03/
RESULTS.md Top-10 fix #2.

internal/connector/issuer/digicert/digicert.go

@@ -39,13 +39,23 @@ import (
 
 	"github.com/shankar0123/certctl/internal/connector/issuer"
 	"github.com/shankar0123/certctl/internal/connector/issuer/asyncpoll"
+	"github.com/shankar0123/certctl/internal/secret"
 )
 
 // Config represents the DigiCert CertCentral issuer connector configuration.
 type Config struct {
 	// APIKey is the CertCentral API key for authentication.
 	// Required. Set via CERTCTL_DIGICERT_API_KEY environment variable.
-	APIKey string `json:"api_key"`
+	//
+	// Type: *secret.Ref (audit fix #6 Phase 3 — Bundle I close).
+	// Wrapping the API key in a Ref means: it never stringifies (Config
+	// marshals as "[redacted]"), the bytes are zeroed after each
+	// Use/WriteTo invocation (defeats heap-dump extraction), and
+	// outbound X-DC-DEVKEY header writes go through Ref.Use so the
+	// staging buffer is short-lived. JSON unmarshal of a string value
+	// populates the Ref via NewRefFromString — operator config files
+	// are unchanged.
+	APIKey *secret.Ref `json:"api_key"`
 
 	// OrgID is the DigiCert organization ID for certificate orders.
 	// Required. Set via CERTCTL_DIGICERT_ORG_ID environment variable.
@@ -149,7 +159,7 @@ func (c *Connector) ValidateConfig(ctx context.Context, rawConfig json.RawMessag
 		return fmt.Errorf("invalid DigiCert config: %w", err)
 	}
 
-	if cfg.APIKey == "" {
+	if cfg.APIKey.IsEmpty() {
 		return fmt.Errorf("DigiCert api_key is required")
 	}
 
@@ -170,7 +180,12 @@ func (c *Connector) ValidateConfig(ctx context.Context, rawConfig json.RawMessag
 	if err != nil {
 		return fmt.Errorf("failed to create API test request: %w", err)
 	}
-	req.Header.Set("X-DC-DEVKEY", cfg.APIKey)
+	if err := cfg.APIKey.Use(func(buf []byte) error {
+		req.Header.Set("X-DC-DEVKEY", string(buf))
+		return nil
+	}); err != nil {
+		return fmt.Errorf("digicert apikey use: %w", err)
+	}
 	req.Header.Set("Content-Type", "application/json")
 
 	resp, err := c.httpClient.Do(req)
@@ -226,7 +241,12 @@ func (c *Connector) IssueCertificate(ctx context.Context, request issuer.Issuanc
 	if err != nil {
 		return nil, fmt.Errorf("failed to create order request: %w", err)
 	}
-	req.Header.Set("X-DC-DEVKEY", c.config.APIKey)
+	if err := c.config.APIKey.Use(func(buf []byte) error {
+		req.Header.Set("X-DC-DEVKEY", string(buf))
+		return nil
+	}); err != nil {
+		return nil, fmt.Errorf("digicert apikey use: %w", err)
+	}
 	req.Header.Set("Content-Type", "application/json")
 
 	resp, err := c.httpClient.Do(req)
@@ -329,7 +349,12 @@ func (c *Connector) RevokeCertificate(ctx context.Context, request issuer.Revoca
 	if err != nil {
 		return fmt.Errorf("failed to create revoke request: %w", err)
 	}
-	req.Header.Set("X-DC-DEVKEY", c.config.APIKey)
+	if err := c.config.APIKey.Use(func(buf []byte) error {
+		req.Header.Set("X-DC-DEVKEY", string(buf))
+		return nil
+	}); err != nil {
+		return fmt.Errorf("digicert apikey use: %w", err)
+	}
 	req.Header.Set("Content-Type", "application/json")
 
 	resp, err := c.httpClient.Do(req)
@@ -415,7 +440,12 @@ func (c *Connector) pollOrderOnce(ctx context.Context, orderID string) (*issuer.
 	if err != nil {
 		return nil, asyncpoll.Failed, fmt.Errorf("failed to create status request: %w", err)
 	}
-	req.Header.Set("X-DC-DEVKEY", c.config.APIKey)
+	if err := c.config.APIKey.Use(func(buf []byte) error {
+		req.Header.Set("X-DC-DEVKEY", string(buf))
+		return nil
+	}); err != nil {
+		return nil, asyncpoll.Failed, fmt.Errorf("digicert apikey use: %w", err)
+	}
 	req.Header.Set("Content-Type", "application/json")
 
 	resp, err := c.httpClient.Do(req)
@@ -518,7 +548,13 @@ func (c *Connector) downloadCertificate(ctx context.Context, certificateID int)
 		err = fmt.Errorf("failed to create download request: %w", reqErr)
 		return
 	}
-	req.Header.Set("X-DC-DEVKEY", c.config.APIKey)
+	if useErr := c.config.APIKey.Use(func(buf []byte) error {
+		req.Header.Set("X-DC-DEVKEY", string(buf))
+		return nil
+	}); useErr != nil {
+		err = fmt.Errorf("digicert apikey use: %w", useErr)
+		return
+	}
 
 	resp, doErr := c.httpClient.Do(req)
 	if doErr != nil {