NestBot(#908): Reaction Management, RAG Search Failures, and Greeting Handling Bugs

This is a sub-issue of #908 (NestBot AI Assistant Implementation).

Several bugs and UX issues were discovered during testing of the NestBot AI Assistant:

1. **Reaction Management Issues**: Bot shows 👀 (eyes) and doesn't show ✅ (tick) reactions for completed tasks, or sometimes only 👀 without ever providing an answer. This creates a confusing user experience where reactions remain stuck and bot status is unclear.

2. **RAG Search Failures**: RAG agent returns "I don't have access to information..." even when relevant chunks exist in the database. Users receive unhelpful responses for valid queries that should be answerable.

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## To Reproduce
Steps to reproduce the behavior:

1. **Greeting Routing Bug:**
- Go to Slack channel
- Send message: "@NestBot Hello"
- See error: Bot incorrectly routes to agents instead of giving friendly greeting

2. **Reaction Management Bug:**
- Go to Slack channel
- Send any query to @NestBot
- Wait for response
- See error: Still 👀 reactions appear (should show ✅)

## Expected behavior

1. **Greeting Handling**: Simple greetings like "Hello", "Hi", "Thanks" should receive friendly, predefined responses without being routed to agents. Only greetings that contain actual questions should be processed by agents.

2. **RAG Search**: RAG agent should try multiple search strategies with different keywords and rephrased queries when initial search yields no results. It should provide helpful guidance when information isn't found, rather than generic "I don't have access" messages.

3. **Reaction Management**:
- Only ✅ (white_check_mark) should remain for successful responses (no 👀 reaction should remain)
- 🤷 (shrugging) reaction should appear when no answer can be generated
- 👀 reaction should be properly removed in all code paths

4. **Test Coverage**: Comprehensive test queries should cover all intent categories (project, chapter, contribution, RAG) and edge cases (greetings, low confidence queries, error scenarios).

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## Are you going to work on fixing this?

- [x] Yes
- [ ] No

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## Screenshots

```
N/A - These are backend/Slack integration bugs that don't have visual screenshots.
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