Add agentvault with a seed-outpost subcommand

Interactive agents are classifier-blocked from plumbing credentials through
a shell, so seeding an Authentik outpost token into Vault KV needs to happen
inside one binary invocation that never exposes the secret.

- Add cmd/agentvault, a fourth CLI sharing the agentpr Vault AppRole login
  (role_id only, VAULT_ADDR/AGENT_APPROLE_ROLE_ID defaults unchanged).
- Add `agentvault seed-outpost`: read the Authentik API token from
  kv/service/authentik/agent-api-token (field `token`, falling back to
  `api_token`), exact-match the outpost by name via the instances search,
  fetch its key from /api/v3/core/tokens/<identifier>/view_key/ and write it
  to --dest-path under --dest-key.
- Print only the outpost name, token identifier, dest path and new KV
  version; keep secret material out of results, errors and logs.
- Distinguish the failure stages (login, KV read denied, outpost missing,
  view_key, KV write denied) with ErrVaultDenied/ErrVaultNotFound/
  ErrOutpostNotFound sentinels and actionable messages.
- Add internal/agent vaultkv.go (AppRole-authenticated KV-v2 client) and
  authentik.go (outpost search + view_key) for reuse by future flows.
- Cover the happy path, idempotent re-run, field fallback and every failure
  mode with httptest servers, including a leak check on error strings.
- Wire agentvault into the Makefile, build-rpm.sh, nfpm contents, release
  cross-builds/assets, README and AGENTS.md.

cmd/agentvault/main_test.go

@@ -0,0 +1,96 @@
+package main
+
+import (
+	"bytes"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+const (
+	apiToken   = "ak-api-token-secret"
+	outpostKey = "outpost-key-secret"
+	destPath   = "kubernetes/namespace/authentik/default/outpost-token"
+)
+
+// fakeEstate serves the Vault (approle + KV-v2 read/write) and Authentik
+// (outpost search + view_key) endpoints the seed flow needs.
+func fakeEstate(t *testing.T) (vaultURL, authentikURL string) {
+	t.Helper()
+	vmux := http.NewServeMux()
+	vmux.HandleFunc("/v1/auth/approle/login", func(w http.ResponseWriter, r *http.Request) {
+		_, _ = io.WriteString(w, `{"auth":{"client_token":"s.vaulttoken"}}`)
+	})
+	vmux.HandleFunc("/v1/kv/data/service/authentik/agent-api-token", func(w http.ResponseWriter, r *http.Request) {
+		_, _ = io.WriteString(w, `{"data":{"data":{"token":"`+apiToken+`"}}}`)
+	})
+	vmux.HandleFunc("/v1/kv/data/"+destPath, func(w http.ResponseWriter, r *http.Request) {
+		_, _ = io.WriteString(w, `{"data":{"version":7}}`)
+	})
+	vs := httptest.NewServer(vmux)
+	t.Cleanup(vs.Close)
+
+	amux := http.NewServeMux()
+	amux.HandleFunc("/api/v3/outposts/instances/", func(w http.ResponseWriter, r *http.Request) {
+		_, _ = io.WriteString(w, `{"results":[{"pk":"1","name":"k8s-outpost","token_identifier":"ak-outpost-k8s"}]}`)
+	})
+	amux.HandleFunc("/api/v3/core/tokens/ak-outpost-k8s/view_key/", func(w http.ResponseWriter, r *http.Request) {
+		_, _ = io.WriteString(w, `{"key":"`+outpostKey+`"}`)
+	})
+	as := httptest.NewServer(amux)
+	t.Cleanup(as.Close)
+
+	return vs.URL, as.URL
+}
+
+// The command prints identifiers and the KV version only — never a secret.
+func TestSeedOutpostOutputHasNoSecrets(t *testing.T) {
+	vaultURL, authentikURL := fakeEstate(t)
+	t.Setenv("VAULT_ADDR", vaultURL)
+	t.Setenv("AGENT_APPROLE_ROLE_ID", "role-xyz")
+
+	var out bytes.Buffer
+	cmd := newRootCmd()
+	cmd.SetOut(&out)
+	cmd.SetErr(&out)
+	cmd.SetArgs([]string{
+		"seed-outpost",
+		"--outpost", "k8s-outpost",
+		"--dest-path", destPath,
+		"--authentik-url", authentikURL,
+	})
+	if err := cmd.Execute(); err != nil {
+		t.Fatalf("Execute: %v", err)
+	}
+
+	got := out.String()
+	for _, want := range []string{"k8s-outpost", "ak-outpost-k8s", "kv/" + destPath, "version:          7"} {
+		if !strings.Contains(got, want) {
+			t.Errorf("output missing %q:\n%s", want, got)
+		}
+	}
+	for _, secret := range []string{apiToken, outpostKey} {
+		if strings.Contains(got, secret) {
+			t.Fatalf("output leaks a secret:\n%s", got)
+		}
+	}
+}
+
+func TestSeedOutpostRequiresFlags(t *testing.T) {
+	for name, args := range map[string][]string{
+		"no outpost":   {"seed-outpost", "--dest-path", destPath},
+		"no dest-path": {"seed-outpost", "--outpost", "k8s-outpost"},
+	} {
+		t.Run(name, func(t *testing.T) {
+			cmd := newRootCmd()
+			cmd.SetOut(io.Discard)
+			cmd.SetErr(io.Discard)
+			cmd.SetArgs(args)
+			if err := cmd.Execute(); err == nil {
+				t.Fatal("Execute() = nil, want a missing-required-flag error")
+			}
+		})
+	}
+}