Add create-plugin command and improve asset management

- Add new create-plugin command to generate plugins from starter template
- Add manifest name subcommand for template context
- Replace hardcoded asset paths with explicit file patterns in embed directive
- Refactor asset build system to use _setup.mk instead of setup.mk
- Update versioning to use pluginctl manifest id for APP_NAME
- Clean up Makefile by removing redundant bundle name definition

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

assets/build/_setup.mk

@@ -0,0 +1,46 @@
+# Ensure that go is installed. Note that this is independent of whether or not a server is being
+# built, since the build script itself uses go.
+ifeq ($(GO),)
+    $(error "go is not available: see https://golang.org/doc/install")
+endif
+
+# Gather build variables to inject into the manifest tool
+BUILD_HASH_SHORT = $(shell git rev-parse --short HEAD)
+BUILD_TAG_LATEST = $(shell git describe --tags --match 'v*' --abbrev=0 2>/dev/null)
+BUILD_TAG_CURRENT = $(shell git tag --points-at HEAD)
+
+# Extract the plugin id from the manifest.
+PLUGIN_ID ?= $(shell pluginctl manifest id)
+ifeq ($(PLUGIN_ID),)
+    $(error "Cannot parse id from $(MANIFEST_FILE)")
+endif
+
+# Extract the plugin version from the manifest.
+PLUGIN_VERSION ?= $(shell pluginctl manifest version)
+ifeq ($(PLUGIN_VERSION),)
+    $(error "Cannot parse version from $(MANIFEST_FILE)")
+endif
+
+# Determine if a server is defined in the manifest.
+HAS_SERVER ?= $(shell pluginctl manifest has_server)
+
+# Determine if a webapp is defined in the manifest.
+HAS_WEBAPP ?= $(shell pluginctl manifest has_webapp)
+
+# Determine if a /public folder is in use
+HAS_PUBLIC ?= $(wildcard public/.)
+
+# Determine if the mattermost-utilities repo is present
+HAS_MM_UTILITIES ?= $(wildcard $(MM_UTILITIES_DIR)/.)
+
+# Store the current path for later use
+PWD ?= $(shell pwd)
+
+# Ensure that npm (and thus node) is installed.
+ifneq ($(HAS_WEBAPP),)
+ifeq ($(NPM),)
+    $(error "npm is not available: see https://www.npmjs.com/get-npm")
+endif
+endif
+
+BUNDLE_NAME ?= $(PLUGIN_ID)-$(PLUGIN_VERSION).tar.gz