fix(env): address P1-P3 review items from code review

- P1: Fix bash completion $cliproxy_profiles scoping — inline profiles
  in env block since variable is only defined at COMP_CWORD=1 scope
- P2: Detect account-based profiles and show specific error message
  instead of generic "not found"
- P2: Show `ccs migrate` hint when unified mode is disabled and settings
  profile resolution fails
- P2: transformToOpenAI omits empty entries at transform time instead of
  relying on output filter (removes fragile coupling)
- P3: Add zsh and auto to --shell completions across all 4 shells; map
  --shell zsh to bash in command handler since syntax is identical
- P3: Auto-detect PowerShell from SHELL containing pwsh on non-Windows
- Tests: 33 pass (+1 pwsh detection test, updated transform assertions)

scripts/completion/ccs.zsh

@@ -128,7 +128,7 @@ _ccs() {
         env)
           _arguments \
             '--format[Output format]:format:(openai anthropic raw)' \
-            '--shell[Shell syntax]:shell:(bash fish powershell)' \
+            '--shell[Shell syntax]:shell:(auto bash zsh fish powershell)' \
             '(- *)'{-h,--help}'[Show help]' \
             '1:profile:($proxy_profiles ${(k)settings_profiles_described})'
           ;;