rotate: 4 SaaS-token/app-signing drivers + data-separated proof tracking

Add gitlab, cloudflare, ghactions (MOCK-ONLY) and appsecret (LIVE-VM) one-file
Rotators, each with a table-driven cutover-proof + leak-check test. gitlab/
cloudflare/ghactions are in-place SaaS-token rolls (self/rotate, value-roll,
sealed-secret overwrite) so RevokeOld is a no-op; ghactions seals via
nacl/box.SealAnonymous (no new go.mod dep). appsecret regenerates a local app
signing secret and rewrites it in place across every target file (atomic, mode-
preserving, redacted errors), discoverable via exact-match app-signing key names
in env.go.

Keep real-rotation code distinct from mock code: how each driver's cutover was
validated lives as DATA in internal/rotate/proofs.go (single source of truth) +
docs/ROTATION-PROOFS.md, surfaced as a PROOF column in `incredigo rotate` so a
MOCK-ONLY driver can never be mistaken for a LIVE-VM one. appsecret proven LIVE-VM
against real local files in the sandbox VM.

82 tests green, -race clean on rotate/sink/vault.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/discover/env.go

@@ -59,8 +59,18 @@ func (e *envScanner) Scan(ctx context.Context, v *vault.Vault) ([]Credential, er
 			// regardless of the secret-ish heuristic: a weak/empty DB password is
 			// exactly the kind of leak we must not miss.
 			source, kind := e.Name(), Kind(KindToken)
+			secretBlob := val
 			if dbSrc := dbSourceForURL(val); dbSrc != "" {
 				source, kind = dbSrc, KindPassword
+			} else if appSigningKey(k) && len(val) >= 12 {
+				// An app SIGNING secret (Django SECRET_KEY, Rails secret_key_base, a
+				// JWT signing secret, …) lives in this very file; tag it Source=
+				// "appsecret" with a self-contained blob carrying the value AND its
+				// file path so the local in-place rotator is driver-ready.
+				source = "appsecret"
+				secretBlob = "appsecret://local/?" + url.Values{
+					"key": {k}, "val": {val}, "path": {p},
+				}.Encode()
 			} else if !looksSecret(k, val) {
 				continue
 			}
@@ -70,7 +80,7 @@ func (e *envScanner) Scan(ctx context.Context, v *vault.Vault) ([]Credential, er
 				Identity: filepath.Base(p) + " / " + k,
 				Location: p,
 				Modified: fi.ModTime(),
-				Secret:   v.Store([]byte(val)),
+				Secret:   v.Store([]byte(secretBlob)),
 			})
 		}
 		f.Close()
@@ -104,6 +114,24 @@ func dbSourceForURL(val string) string {
 	return ""
 }
 
+// appSigningKeys are the env-var names that denote a local application SIGNING secret
+// — the kind the appsecret rotator can regenerate in place. Matched case-insensitively
+// and exactly (so STRIPE_API_KEY / DB_PASSWORD stay generic env secrets, not appsecrets).
+var appSigningKeys = map[string]bool{
+	"secret_key":        true, // Django / Flask
+	"secret_key_base":   true, // Rails
+	"django_secret_key": true,
+	"flask_secret_key":  true,
+	"app_key":           true, // Laravel
+	"app_secret":        true,
+	"jwt_secret":        true,
+	"session_secret":    true,
+	"nextauth_secret":   true,
+	"rails_secret":      true,
+}
+
+func appSigningKey(key string) bool { return appSigningKeys[strings.ToLower(key)] }
+
 func looksSecret(key, val string) bool {
 	lk := strings.ToLower(key)
 	for _, h := range sensitiveHints {