feat: add AskSage LLM provider integration

Implement AskSageAdapter using the official asksageclient SDK to support
AskSage as an LLM provider option. This enables users to leverage
AskSage's API with configurable email, API key, and model settings.

- Add AskSageAdapter class with async support via thread pool
- Update Settings to include asksage_email, asksage_api_key, asksage_model
- Extend llm_mode literal to include "asksage" option
- Update dependency injection to instantiate AskSageAdapter when configured
- Remove completed OPENAI_INTEGRATION_PLAN.md
- Update requirements.txt with full dependency list including asksageclient

.env.example

@@ -1,4 +1,4 @@
-# LLM Mode: "local", "remote", or "openai"
+# LLM Mode: "local", "remote", "openai", or "asksage"
 LLM_MODE=local
 
 # Remote LLM Configuration (required if LLM_MODE=remote)
@@ -8,3 +8,8 @@ LLM_REMOTE_TOKEN=
 # OpenAI Configuration (required if LLM_MODE=openai)
 OPENAI_API_KEY=sk-your-api-key-here
 OPENAI_MODEL=gpt-4o-mini
+
+# AskSage Configuration (required if LLM_MODE=asksage)
+ASKSAGE_EMAIL=your.email@example.com
+ASKSAGE_API_KEY=your-api-key-here
+ASKSAGE_MODEL=gpt-4o