refactor(sdr-agent): reuse agent.start() in tui command and fix mock mode

- Replace duplicated setup code in tui command with agent.start(mock_mode=mock)
- Fix mock mode to use MockLLMProvider instead of llm=None
- Add demo_contacts.json sample data for template testing
- Untrack .claude/settings.json and add to .gitignore

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -69,6 +69,7 @@ exports/*
 
 .claude/settings.local.json
 .claude/skills/ship-it/
+.claude/settings.json
 
 .venv