Add proactive context injection

[Jmeg8r]

Implement the ability for Cortex to proactively inject relevant context into sessions without being asked. Based on the current task, Cortex should surface related decisions, conventions, past solutions, and potential pitfalls.

Acceptance Criteria

• Context relevance scoring based on current session activity
• Injection timing logic (when to surface context vs. stay silent)
• Integration with Claude Code session hooks
• User control: ability to adjust verbosity and frequency
• Tests covering injection relevance and timing