.env.example: say why GIT_USER_* must be set

Both keys shipped empty with no explanation, so they read as optional. They
are not: ~/.gitconfig is not on a persistent mount, so with these unset every
repo in the container fails "Author identity unknown" on first commit after
every recreate — and an agent asked to commit then infers an identity from
git log and picks the wrong one. Observed today across three repos on one
machine (two wrong-address commits, plus older `pi <pi@devbox>` fossils from
earlier sessions guessing the same way).

Also records that the address is per-machine (corporate vs private), so it
belongs in the per-machine .env rather than a skill or a repo-local override.

.env.example

@@ -52,6 +52,13 @@ SSH_KEY_PATH=~/.ssh
 # DEVBOX_ATELIER=1
 
 # ── Git Configuration ────────────────────────────────────────────────
+# Set BOTH. If unset, every repo inside the container fails with
+# "Author identity unknown" on first commit, and an agent asked to commit
+# will guess an identity from git log — often the wrong one. The e-mail is
+# per-machine (work machines use the corporate address, personal machines the
+# private one), so it belongs in this per-machine .env, never in a skill or a
+# repo-local override. Consumed by entrypoint-user.sh -> ~/.gitconfig, which is
+# NOT persistent across container recreate — this file is the source of truth.
 GIT_USER_NAME=
 GIT_USER_EMAIL=