Add demo app with mock, Ollama, OpenAI, and Anthropic provider support

Example iOS app that seeds a task management SQLite database and
renders SwiftDBAI's DataChatView. Ships with a mock LLM for offline
use and an OllamaWithSystemPrompt wrapper that fixes AnyLanguageModel's
Ollama adapter dropping system instructions.

Tested successfully with:
- DemoLanguageModel (mock, pattern-matched SQL)
- Ollama qwen3-coder-30b (local, real SQL generation)
- OpenAI gpt-4o-mini (JOINs, GROUP BY, multi-turn)
- Anthropic claude-haiku-4.5 (complex aggregations)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Example/SwiftDBAIDemo/project.yml

@@ -0,0 +1,26 @@
+name: SwiftDBAIDemo
+options:
+  bundleIdPrefix: com.swiftdbai.demo
+  deploymentTarget:
+    iOS: "17.0"
+  xcodeVersion: "16.0"
+  createIntermediateGroups: true
+packages:
+  SwiftDBAI:
+    path: ../..
+targets:
+  SwiftDBAIDemo:
+    type: application
+    platform: iOS
+    sources:
+      - SwiftDBAIDemo
+    settings:
+      base:
+        PRODUCT_BUNDLE_IDENTIFIER: com.swiftdbai.demo
+        MARKETING_VERSION: "1.0"
+        CURRENT_PROJECT_VERSION: "1"
+        SWIFT_VERSION: "6.0"
+        INFOPLIST_GENERATION: true
+        GENERATE_INFOPLIST_FILE: true
+    dependencies:
+      - package: SwiftDBAI