Make the Vault gitea creds path selectable

agentpr always read gitea/creds/unkin-agent from a bare const, so a service
like repospawner could not run it as its own Gitea identity.

- Replace the GiteaCredsPath const with a function: GITEA_CREDS_PATH when set,
  otherwise gitea/creds/<AGENT_LOGIN>. Unset env still resolves to
  gitea/creds/unkin-agent, so existing callers are unchanged.
- Thread the creds path through fetchGiteaToken/readGiteaCreds instead of
  reading a package-level const, and report it in the error messages.
- Make agentpr's help text login-agnostic and document both variables.

internal/agent/vault.go

@@ -49,9 +49,10 @@ func approleLogin(vaultAddr, roleID string) (string, error) {
 	return out.Auth.ClientToken, nil
 }
 
-// readGiteaCreds reads the Gitea creds secret and returns the token field.
-func readGiteaCreds(vaultAddr, clientToken string) (string, error) {
-	url := strings.TrimRight(vaultAddr, "/") + "/v1/" + GiteaCredsPath
+// readGiteaCreds reads the Gitea creds secret at credsPath and returns the
+// token field.
+func readGiteaCreds(vaultAddr, clientToken, credsPath string) (string, error) {
+	url := strings.TrimRight(vaultAddr, "/") + "/v1/" + credsPath
 	req, err := http.NewRequest(http.MethodGet, url, nil)
 	if err != nil {
 		return "", err
@@ -60,12 +61,12 @@ func readGiteaCreds(vaultAddr, clientToken string) (string, error) {
 
 	resp, err := httpClient.Do(req)
 	if err != nil {
-		return "", fmt.Errorf("vault read %s: %w", GiteaCredsPath, err)
+		return "", fmt.Errorf("vault read %s: %w", credsPath, err)
 	}
 	defer func() { _ = resp.Body.Close() }()
 	data, _ := io.ReadAll(resp.Body)
 	if resp.StatusCode != http.StatusOK {
-		return "", fmt.Errorf("vault read %s: HTTP %d: %s", GiteaCredsPath, resp.StatusCode, strings.TrimSpace(string(data)))
+		return "", fmt.Errorf("vault read %s: HTTP %d: %s", credsPath, resp.StatusCode, strings.TrimSpace(string(data)))
 	}
 
 	var out struct {
@@ -74,10 +75,10 @@ func readGiteaCreds(vaultAddr, clientToken string) (string, error) {
 		} `json:"data"`
 	}
 	if err := json.Unmarshal(data, &out); err != nil {
-		return "", fmt.Errorf("vault read %s: decoding response: %w", GiteaCredsPath, err)
+		return "", fmt.Errorf("vault read %s: decoding response: %w", credsPath, err)
 	}
 	if out.Data.Token == "" {
-		return "", fmt.Errorf("vault read %s: no token field in secret", GiteaCredsPath)
+		return "", fmt.Errorf("vault read %s: no token field in secret", credsPath)
 	}
 	return out.Data.Token, nil
 }