Add updateassets command with webapp code detection

- Add updateassets command to update plugin files from embedded assets
- Only include webapp assets if plugin manifest indicates webapp code presence
- Compare file contents before updating to avoid unnecessary writes
- Display count of updated files in completion message

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

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

assets/.golangci.yml

@@ -0,0 +1,48 @@
+run:
+  timeout: 5m
+  modules-download-mode: readonly
+
+linters-settings:
+  gofmt:
+    simplify: true
+  goimports:
+    local-prefixes: github.com/mattermost/mattermost-starter-template
+  govet:
+    check-shadowing: true
+    enable-all: true
+    disable:
+      - fieldalignment
+  misspell:
+    locale: US
+
+linters:
+  disable-all: true
+  enable:
+    - bodyclose
+    - errcheck
+    - gocritic
+    - gofmt
+    - goimports
+    - gosec
+    - gosimple
+    - govet
+    - ineffassign
+    - misspell
+    - nakedret
+    - revive
+    - staticcheck
+    - stylecheck
+    - typecheck
+    - unconvert
+    - unused
+    - whitespace
+
+issues:
+  exclude-rules:
+    - path: server/configuration.go
+      linters:
+        - unused
+    - path: _test\.go
+      linters:
+        - bodyclose
+        - scopelint # https://github.com/kyoh86/scopelint/issues/4