docs: add MLflow tracking as example of AI provider agnosticism principle

## Summary
Add the MLflow tracking implementation as a concrete example in the AI provider agnosticism principle documentation.

## Context
We just implemented truly provider-agnostic MLflow tracking in PR #1328 that demonstrates the principle perfectly by:
- Avoiding N×M complexity (N providers × M patterns)
- Focusing on extracting actual commands rather than provider-specific formatting
- Using a single parser (`parse_session.py`) for all AI assistants

## What to Document

### In `knowledge/principles/ai-provider-agnosticism.md`

Add a new section showing MLflow tracking as a real-world example:

```markdown
## Implementation Example: MLflow Session Tracking

The MLflow tracking system demonstrates provider agnosticism by extracting actual commands from transcripts rather than maintaining provider-specific patterns:

**Anti-pattern (N×M complexity):**
- Different regex patterns for each provider
- Provider detection logic
- Maintenance burden grows with each new AI assistant

**Correct pattern (provider-agnostic):**
- Single parser looks for actual `git`, `gh`, and bash commands
- No provider detection needed
- Works automatically with any AI assistant

See: `tracking/parse_session.py` - Extracts real commands regardless of AI formatting
```

## Benefits
- Shows a concrete implementation of the principle
- Demonstrates how to avoid the N×M problem
- Provides a reference for future provider-agnostic implementations

## Related
- PR #1328: MLflow provider-agnostic implementation
- Issue #1326: Original MLflow tracking feature request

Start in knowledge/principles/ai-provider-agnosticism.md and review tracking/parse_session.py to confirm the commands and provider-agnostic behavior described. Add the MLflow Session Tracking example with the anti-pattern, correct pattern, and reference requested in the issue. Done means the principle documentation includes this concrete example accurately.

Written by the indexing model from the issue text.