.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=
 

README.md

@@ -882,12 +882,31 @@ cat ~/.ssh-local/mypeer_devbox_ed25519.pub
 # ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIEXAMPLE0000EXAMPLE0000EXAMPLE0000ex devbox-0d11ec7731c7
 ```
 
-**2. On the peer** — authorize it narrowly rather than bare:
+**2. On the peer** — append that public key to `~/.ssh/authorized_keys` **of
+the account you will log in as** (the `User` from step 3), narrowly authorized
+rather than bare:
 
-```
+```bash
+mkdir -p ~/.ssh && chmod 700 ~/.ssh
+cat >> ~/.ssh/authorized_keys <<'KEY'
 from="192.168.1.0/24,192.168.4.0/24,10.8.0.7",restrict ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIEXAMPLE0000EXAMPLE0000EXAMPLE0000ex devbox-mymachine
+KEY
+chmod 600 ~/.ssh/authorized_keys
 ```
 
+Both lines are safe on a peer that is already set up: `mkdir -p` is a no-op
+when the directory exists, the `chmod`s only tighten, and appending never
+touches keys already listed. Use `>>`, never `>` — one stray truncation
+revokes every other key on that account. The options prefix must sit on the
+**same physical line** as the key, comma-separated with no spaces: a paste
+that wrapped is the likeliest reason a key that looks right is refused.
+`ssh-copy-id` cannot add that prefix, so append by hand (or let it copy the
+bare key and edit the line afterwards). If authentication still fails with no
+clear reason, suspect permissions — sshd's `StrictModes` silently ignores
+`authorized_keys` when the home directory, `~/.ssh` or the file itself is
+group- or world-writable, and says why only in the peer's own log
+(`journalctl -u ssh`, `/var/log/auth.log`).
+
 `restrict` disables pty, agent/X11 and port forwarding; append
 `port-forwarding` and `permitopen="127.0.0.1:<port>"` after it if you need one
 specific tunnel. `from=` must list the **host's** addresses, not the