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.

internal/agent/authentik.go

@@ -0,0 +1,93 @@
+package agent
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+)
+
+// ErrOutpostNotFound marks a search that returned no exactly-named outpost.
+var ErrOutpostNotFound = errors.New("outpost not found")
+
+// AuthentikClient talks to the Authentik REST API with a bearer API token. The
+// internal CA is in the OS trust store, so the default transport suffices.
+type AuthentikClient struct {
+	BaseURL string
+	Token   string
+	HTTP    *http.Client
+}
+
+// NewAuthentikClient builds a client for the given Authentik base URL.
+func NewAuthentikClient(baseURL, token string) *AuthentikClient {
+	return &AuthentikClient{BaseURL: strings.TrimRight(baseURL, "/"), Token: token, HTTP: httpClient}
+}
+
+// Outpost is the subset of Authentik's outpost object we need.
+type Outpost struct {
+	PK              string `json:"pk"`
+	Name            string `json:"name"`
+	TokenIdentifier string `json:"token_identifier"`
+}
+
+// get issues an authenticated GET and decodes into out. Error text never
+// includes a successful response body, which may carry key material.
+func (c *AuthentikClient) get(path string, out any) error {
+	req, err := http.NewRequest(http.MethodGet, c.BaseURL+path, nil)
+	if err != nil {
+		return err
+	}
+	req.Header.Set("Authorization", "Bearer "+c.Token)
+	req.Header.Set("Accept", "application/json")
+
+	resp, err := c.HTTP.Do(req)
+	if err != nil {
+		return fmt.Errorf("authentik GET %s: %w", path, err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+	data, _ := io.ReadAll(resp.Body)
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return fmt.Errorf("authentik GET %s: HTTP %d: %s", path, resp.StatusCode, strings.TrimSpace(string(data)))
+	}
+	if err := json.Unmarshal(data, out); err != nil {
+		return fmt.Errorf("authentik GET %s: decoding response: %w", path, err)
+	}
+	return nil
+}
+
+// FindOutpost searches outpost instances and returns the one whose name matches
+// exactly (search is a substring match, so the exact name is re-checked here).
+func (c *AuthentikClient) FindOutpost(name string) (Outpost, error) {
+	var out struct {
+		Results []Outpost `json:"results"`
+	}
+	path := "/api/v3/outposts/instances/?search=" + url.QueryEscape(name)
+	if err := c.get(path, &out); err != nil {
+		return Outpost{}, err
+	}
+	for _, o := range out.Results {
+		if o.Name == name {
+			return o, nil
+		}
+	}
+	return Outpost{}, fmt.Errorf("authentik outpost %q: %w (searched %d result(s))", name, ErrOutpostNotFound, len(out.Results))
+}
+
+// TokenKey returns the key behind a token identifier
+// (GET /api/v3/core/tokens/<identifier>/view_key/).
+func (c *AuthentikClient) TokenKey(identifier string) (string, error) {
+	var out struct {
+		Key string `json:"key"`
+	}
+	path := "/api/v3/core/tokens/" + url.PathEscape(identifier) + "/view_key/"
+	if err := c.get(path, &out); err != nil {
+		return "", err
+	}
+	if out.Key == "" {
+		return "", fmt.Errorf("authentik view_key for %q: response has no key field", identifier)
+	}
+	return out.Key, nil
+}