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.ps1

@@ -23,7 +23,7 @@ Register-ArgumentCompleter -CommandName ccs -ScriptBlock {
     $updateFlags = @('--force', '--beta', '--dev', '--help', '-h')
     $envFlags = @('--format', '--shell', '--help', '-h')
     $envFormats = @('openai', 'anthropic', 'raw')
-    $envShells = @('bash', 'fish', 'powershell')
+    $envShells = @('auto', 'bash', 'zsh', 'fish', 'powershell')
     $shellCompletionFlags = @('--bash', '--zsh', '--fish', '--powershell')
     $listFlags = @('--verbose', '--json')
     $removeFlags = @('--yes', '-y')