Minor cleanups: default TZ to UTC, shellcheck directive, comment clarity (#21)

- Default timezone to UTC instead of America/New_York
- Add shellcheck shell=bash directive to .zshrc
- Extract docker filter label into local variable in cmd_down
- Clarify mount filter comment (target paths immune to project name changes)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

devcontainer.json

@@ -4,7 +4,7 @@
   "build": {
     "dockerfile": "Dockerfile",
     "args": {
-      "TZ": "${localEnv:TZ:America/New_York}",
+      "TZ": "${localEnv:TZ:UTC}",
       "GIT_DELTA_VERSION": "0.18.2",
       "ZSH_IN_DOCKER_VERSION": "1.2.1"
     }