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

deploy/README.md

@@ -26,6 +26,20 @@ Alternatively, if they have cloned the repository, they can execute:
 bash deploy/install.sh
 ```
 
+### 2. Custom Fork / Private Mirror Installations
+If you run a private mirror or fork, you can override the source URLs during installation using environment variables:
+
+```bash
+# Installing from a custom mirror (pipe prepends must be applied to the bash command)
+curl -fsSL https://my-mirror.example.com/.../install.sh \
+  | MAM_REPO_URL=https://my-mirror.example.com/me/multi-agent-mux.git \
+    MAM_ARCHIVE_URL=https://my-mirror.example.com/me/multi-agent-mux/archive/main.tar.gz \
+    bash
+
+# Updating an existing workspace against a mirror
+MAM_INSTALLER_URL=https://my-mirror.example.com/.../install.sh bash deploy/update.sh
+```
+
 ### 2. Registering as a Workspace Plugin
 To register these skills globally or for a specific workspace:
 *   **Workspace Level**: Copy the `.agents/` folder into your project root.