fix(tui): make prompt suggestion configurable (opt-in, default off)

- Add prompt_suggestion: Option<bool> config field with
  prompt_suggestion_enabled() accessor (defaults to false)
- Guard suggestion generation behind the config check
- Use config.default_model() (provider-aware) instead of hardcoded
  deepseek-v4-flash to avoid cross-provider data egress
- Document in config.example.toml

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

config.example.toml

@@ -144,6 +144,7 @@ memory_path = "~/.codewhale/memory.md"
 allow_shell = true
 approval_policy = "on-request" # on-request | untrusted | never
 sandbox_mode = "workspace-write" # read-only | workspace-write | danger-full-access | external-sandbox
+# prompt_suggestion = true  # opt-in: show ghost-text follow-up question in composer after each turn
 
 # Typed permission rules live in a sibling `permissions.toml` file, not in
 # config.toml. This schema slice is ask-only and is parsed for follow-up