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.
unkin-agent