refactor: rename project to firebase-auth-cli with fire-auth binary (#1)

The old name "fbadmin" implied broader Firebase Admin SDK coverage.
This tool only does Firebase Authentication, so the new names better
reflect its scope:

- Package: firebase-auth-cli
- Binary: fire-auth
- Env vars: FIRE_AUTH_*
- Config dir/file: fire-auth / .fire-auth.toml

fix: add rust-toolchain.toml to pin Rust 1.94 for CI

The aarch64-unknown-linux-gnu runner ships rustc 1.93 which doesn't
satisfy rust-version = "1.94". Pin the toolchain so cargo-dist
installs the correct version.

Also adds docs/RELEASING.md, AGENTS.md, and fixes README license.