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.fish

@@ -176,7 +176,7 @@ complete -c ccs -n '__fish_seen_subcommand_from doctor' -s h -l help -d 'Show he
 complete -c ccs -n '__fish_seen_subcommand_from env' -l format -d 'Output format'
 complete -c ccs -n '__fish_seen_subcommand_from env; and __fish_seen_argument -l format' -a 'openai anthropic raw' -d 'Format'
 complete -c ccs -n '__fish_seen_subcommand_from env' -l shell -d 'Shell syntax'
-complete -c ccs -n '__fish_seen_subcommand_from env; and __fish_seen_argument -l shell' -a 'bash fish powershell' -d 'Shell'
+complete -c ccs -n '__fish_seen_subcommand_from env; and __fish_seen_argument -l shell' -a 'auto bash zsh fish powershell' -d 'Shell'
 complete -c ccs -n '__fish_seen_subcommand_from env' -s h -l help -d 'Show help for env command'
 
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