Add shared-machine multi-user setup with per-user isolation via SIGNUM

For machines where multiple users share one OS account. Each user gets
isolated containers, config, and named volumes by running docker compose
from their own directory with a unique SIGNUM in .env.

.env.shared.example

@@ -0,0 +1,27 @@
+# ── Shared machine setup ─────────────────────────────────────────────
+# Your corporate signum / username (REQUIRED)
+# This isolates your container, config, and data from other users.
+SIGNUM=your-signum-here
+
+# ── Provider ─────────────────────────────────────────────────────────
+OPENCODE_PROVIDER=amazon-bedrock
+OPENCODE_MODEL=amazon-bedrock/eu.anthropic.claude-opus-4-6-v1
+AWS_REGION=eu-west-1
+AWS_PROFILE=default
+
+# ── Git ──────────────────────────────────────────────────────────────
+GIT_USER_NAME=Your Name
+GIT_USER_EMAIL=your.name@example.com
+
+# ── Paths (adjust to your layout) ───────────────────────────────────
+# Default: ~/src mounted as /workspace
+# WORKSPACE_PATH=~/src
+
+# SSH keys — defaults to shared ~/.ssh
+# If you have per-user keys: SSH_KEY_PATH=~/<signum>/.ssh
+# SSH_KEY_PATH=~/.ssh
+
+# ── Locale (defaults to en_US.UTF-8) ────────────────────────────────
+# LANG=sv_SE.UTF-8
+# LANGUAGE=sv_SE:sv
+# LC_ALL=sv_SE.UTF-8