Merge pull request #16 from vyos/T8595-add-agents-md

general: T8595: add AGENTS.md

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+# AGENTS.md
+
+## Project purpose
+
+NSS (Name Service Switch) module that maps `getpwnam()` / `getpwuid()` lookups for arbitrary usernames onto a single named local account. Used in conjunction with `pam_radius`/TACACS+/etc. to make remote-authenticated users appear as a single configured local user without provisioning each one in `/etc/passwd`. Originally written by Cumulus Networks (Dave Olson, 2017); maintained by VyOS for VyOS images.
+
+## Tech stack
+
+- C; GNU `make` (single `Makefile`).
+- Two NSS plugins built from this tree:
+  - `libnss_mapname.so.2` (name lookup — last in nsswitch.conf chain).
+  - `libnss_mapuid.so.2` (UID lookup — first in chain so mapped UIDs resolve to the mapped name).
+- Manpages: `nss_mapuser.5` (config-file format), `nss_mapuser.8` (operator).
+- Debian packaging under `debian/` (binary package: `vyos-libnss-mapuser`).
+- Legacy Jenkinsfile present (Cumulus-era CI; not used by VyOS).
+
+## Build / test / run
+
+```
+make
+sudo make install
+# Debian package
+dpkg-buildpackage -uc -us -tc -b
+```
+
+Configuration: `/etc/nss_mapuser.conf` — defines the single account to which all unknown names map. Plugins are wired in `/etc/nsswitch.conf` per the README.
+
+No automated test suite.
+
+## Repository layout
+
+- `map_common.[ch]` — shared mapping logic.
+- `nss_mapname.c` — name-lookup plugin source.
+- `nss_mapuid.c` — UID-lookup plugin source.
+- `nss_mapuser.conf` — example config file.
+- `nss_mapuser.5`, `nss_mapuser.8` — manpages.
+- `debian/` — Debian packaging (`vyos-libnss-mapuser` binary, postinst/prerm hooks, lintian overrides, symbols).
+- `Makefile`, `LICENSE` (GPL-2), `AUTHORS`, `README`, `Jenkinsfile` (legacy Cumulus CI).
+
+## Cross-repo context
+
+One of the §3.6 authentication libraries (`libnss-*`, `libpam-*`, `libtacplus-map`). Pre-dependency of `vyos-1x` for RADIUS/TACACS+ auth flows. Native-package category — built and shipped to `packages.vyos.net` via the internal build-packages workflow, then consumed at ISO assembly time by `vyos/vyos-build`.
+
+## Conventions
+
+- Commit / PR title format: `component: T12345: description` (Phorge task ID at https://vyos.dev mandatory). Enforced by `vyos/.github` reusable workflows where consumed.
+- Branch model: `current` (rolling), `circinus` (1.5 LTS), `sagitta` (1.4 LTS), `equuleus` (1.3 LTS).
+- Treat as upstream-vendored — keep diffs against the Cumulus baseline minimal.
+
+## Notes for future contributors
+
+- The NSS module ordering matters — `libnss_mapuid` must be **first** and `libnss_mapname` **last** in `nsswitch.conf`. The README explains why; don't reverse them.
+- `passwd` field is forced to `'x'` so PAM auth on the base account is impossible — only the mapped accounts authenticate (typically via `pam_radius`).
+- License: GPL-2-or-later. Original copyright Cumulus Networks; VyOS additions (2020+) layered on top.