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.

cmd/agentpr/main.go

@@ -1,7 +1,7 @@
-// Command agentpr manages Gitea pull requests and comments as the unkin-agent
-// user. It obtains a scoped Gitea token from Vault (AppRole login, then reads
-// gitea/creds/unkin-agent) so actions are attributed to the agent rather than
-// to whoever runs the tool.
+// Command agentpr manages Gitea pull requests and comments as an agent user. It
+// obtains a scoped Gitea token from Vault (AppRole login, then reads
+// gitea/creds/<AGENT_LOGIN>, or GITEA_CREDS_PATH when set) so actions are
+// attributed to that agent rather than to whoever runs the tool.
 //
 //	agentpr pr create --repo owner/repo --base main --head feature --title T --body B
 //	agentpr pr comment --repo owner/repo --pr 12 --body "..."
@@ -33,8 +33,8 @@ func main() {
 func newRootCmd() *cobra.Command {
 	root := &cobra.Command{
 		Use:          "agentpr",
-		Short:        "Manage Gitea PRs and comments as the unkin-agent user.",
-		Long:         "agentpr manages Gitea pull requests and comments as unkin-agent, using a\nGitea token minted from Vault (AppRole login + gitea/creds/unkin-agent).",
+		Short:        "Manage Gitea PRs and comments as an agent user.",
+		Long:         "agentpr manages Gitea pull requests and comments as an agent user, using a\nGitea token minted from Vault (AppRole login + gitea/creds/<AGENT_LOGIN>).\nSet AGENT_LOGIN to act as another agent identity, or GITEA_CREDS_PATH to name\nthe Vault creds path outright.",
 		Version:      version,
 		SilenceUsage: true,
 	}
@@ -146,7 +146,7 @@ func newPRCommentCmd() *cobra.Command {
 func newWhoamiCmd() *cobra.Command {
 	return &cobra.Command{
 		Use:          "whoami",
-		Short:        "Print the authenticated Gitea login (should be unkin-agent)",
+		Short:        "Print the authenticated Gitea login (the identity PRs are opened as)",
 		SilenceUsage: true,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			c, err := client()