feat(deploy): parameterize distribution URLs via MAM_*_URL env vars

- install.sh: REPO_URL and ARCHIVE_URL now use ${MAM_REPO_URL:-...} and ${MAM_ARCHIVE_URL:-...}
- update.sh: INSTALLER_URL now uses ${MAM_INSTALLER_URL:-...} with env-inheritance comment
- .env.example: add MAM_REPO_URL / MAM_ARCHIVE_URL / MAM_INSTALLER_URL section with usage notes
- deploy/README.md: add Custom Fork / Private Mirror installation examples

.env.example

@@ -75,3 +75,23 @@
 # Directory for delegate-job audit logs (sits beside .mam/jobs/).
 #default: <cwd>/.mam/delegate_job_logs
 # DELEGATE_JOB_LOGS_DIR=/path/to/workspace/.mam/delegate_job_logs
+
+# ==============================================================================
+# deploy / distribution source (for forks/mirrors)
+# ==============================================================================
+# Note: These variables are read from the execution environment by deployment scripts.
+# Since deploy/install.sh runs before .env exists, you must pass them via export
+# or prepended variables (e.g. MAM_REPO_URL=... bash deploy/install.sh).
+# If you run a private mirror, we strongly recommend configuring all three variables.
+
+# Distribution repository URL (cloned during recovery steps).
+#default: https://git.godopu.com/tmpl/multi-agent-mux.git
+# MAM_REPO_URL=https://git.godopu.com/tmpl/multi-agent-mux.git
+
+# Distribution archive download URL (used for bootstrap extraction).
+#default: https://git.godopu.com/tmpl/multi-agent-mux/archive/main.tar.gz
+# MAM_ARCHIVE_URL=https://git.godopu.com/tmpl/multi-agent-mux/archive/main.tar.gz
+
+# Distribution update/installer script URL.
+#default: https://git.godopu.com/tmpl/multi-agent-mux/raw/branch/main/deploy/install.sh
+# MAM_INSTALLER_URL=https://git.godopu.com/tmpl/multi-agent-mux/raw/branch/main/deploy/install.sh