fix: rename add-project to add_project in mux module (#765)

Terraform variable names should use underscores, not hyphens. Renames
the `add-project` variable to `add_project` in the mux module.

**Changes:**
- `main.tf`: Renamed variable declaration and references
- `README.md`: Updated example usage

Bumped version: 1.3.0 → 1.3.1

---
Generated with [Mux](https://mux.coder.com) using Claude

registry/coder/modules/mux/README.md

@@ -14,7 +14,7 @@ Automatically install and run [Mux](https://github.com/coder/mux) in a Coder wor
 module "mux" {
   count    = data.coder_workspace.me.start_count
   source   = "registry.coder.com/coder/mux/coder"
-  version  = "1.3.0"
+  version  = "1.3.1"
   agent_id = coder_agent.main.id
 }
 ```
@@ -37,7 +37,7 @@ module "mux" {
 module "mux" {
   count    = data.coder_workspace.me.start_count
   source   = "registry.coder.com/coder/mux/coder"
-  version  = "1.3.0"
+  version  = "1.3.1"
   agent_id = coder_agent.main.id
 }
 ```
@@ -48,7 +48,7 @@ module "mux" {
 module "mux" {
   count    = data.coder_workspace.me.start_count
   source   = "registry.coder.com/coder/mux/coder"
-  version  = "1.3.0"
+  version  = "1.3.1"
   agent_id = coder_agent.main.id
   # Default is "latest"; set to a specific version to pin
   install_version = "0.4.0"
@@ -63,9 +63,9 @@ Start Mux with `mux server --add-project /path/to/project`:
 module "mux" {
   count       = data.coder_workspace.me.start_count
   source      = "registry.coder.com/coder/mux/coder"
-  version     = "1.3.0"
+  version     = "1.3.1"
   agent_id    = coder_agent.main.id
-  add-project = "/path/to/project"
+  add_project = "/path/to/project"
 }
 ```
 
@@ -78,7 +78,7 @@ The module parses quoted values, so grouped arguments remain intact.
 module "mux" {
   count                = data.coder_workspace.me.start_count
   source               = "registry.coder.com/coder/mux/coder"
-  version              = "1.3.0"
+  version              = "1.3.1"
   agent_id             = coder_agent.main.id
   additional_arguments = "--open-mode pinned --add-project '/workspaces/my repo'"
 }
@@ -90,7 +90,7 @@ module "mux" {
 module "mux" {
   count    = data.coder_workspace.me.start_count
   source   = "registry.coder.com/coder/mux/coder"
-  version  = "1.3.0"
+  version  = "1.3.1"
   agent_id = coder_agent.main.id
   port     = 8080
 }
@@ -104,7 +104,7 @@ Force a specific package manager instead of auto-detection:
 module "mux" {
   count           = data.coder_workspace.me.start_count
   source          = "registry.coder.com/coder/mux/coder"
-  version         = "1.3.0"
+  version         = "1.3.1"
   agent_id        = coder_agent.main.id
   package_manager = "pnpm" # or "npm", "bun"
 }
@@ -118,7 +118,7 @@ Use a private or mirrored npm registry:
 module "mux" {
   count        = data.coder_workspace.me.start_count
   source       = "registry.coder.com/coder/mux/coder"
-  version      = "1.3.0"
+  version      = "1.3.1"
   agent_id     = coder_agent.main.id
   registry_url = "https://npm.pkg.github.com"
 }
@@ -132,7 +132,7 @@ Run an existing copy of Mux if found, otherwise install from npm:
 module "mux" {
   count      = data.coder_workspace.me.start_count
   source     = "registry.coder.com/coder/mux/coder"
-  version    = "1.3.0"
+  version    = "1.3.1"
   agent_id   = coder_agent.main.id
   use_cached = true
 }
@@ -146,7 +146,7 @@ Run without installing from the network (requires Mux to be pre-installed):
 module "mux" {
   count    = data.coder_workspace.me.start_count
   source   = "registry.coder.com/coder/mux/coder"
-  version  = "1.3.0"
+  version  = "1.3.1"
   agent_id = coder_agent.main.id
   install  = false
 }

registry/coder/modules/mux/main.tf

@@ -49,7 +49,7 @@ variable "log_path" {
   default     = "/tmp/mux.log"
 }
 
-variable "add-project" {
+variable "add_project" {
   type        = string
   description = "Optional path to add/open as a project in Mux on startup."
   default     = null
@@ -165,7 +165,7 @@ resource "coder_script" "mux" {
     VERSION : var.install_version,
     PORT : var.port,
     LOG_PATH : var.log_path,
-    ADD_PROJECT : var.add-project == null ? "" : var.add-project,
+    ADD_PROJECT : var.add_project == null ? "" : var.add_project,
     ADDITIONAL_ARGUMENTS : var.additional_arguments,
     INSTALL_PREFIX : var.install_prefix,
     OFFLINE : !var.install,