docs(agents): note nushell is the interactive shell

agents/AGENTS.md

@@ -123,6 +123,9 @@ For an impactful change, follow these steps in order, scaled to its risk:
   `nix profile install`, global `npm`/`pip`). Use `nix shell nixpkgs#<pkg> -c <cmd>` or `nix run`
   for one-off tools, and the project's existing toolchain (e.g. its flake, `uv`, `pnpm`) for its
   dependencies; ask before creating a flake or installing another way.
+- The user's interactive shell is **nushell**; bash is only started from within nushell,
+  occasionally, for tasks nushell can't do. Put shell env, aliases, and per-session secrets in the
+  Nushell config (or `nushell-secrets.nu`), not in `bashrc`/`zshrc`.
 - Use `gh` for authorized GitHub operations; keep SSH for GitHub Git remotes.
 - Code layout: `~/codes` = personal, `~/work` = work code, `~/src/<repo>` = source checkouts.
 - Publish only to repositories that are already public; treat everything else as confidential.