feat: Diff mode for update — show what changed in BRAIN.md

[Hrithik-Gavankar]

Summary

When running engineer-brain update, show a clear, colored diff of what changed in BRAIN.md compared to the previous state — making it obvious what the brain learned.

Motivation

• Currently update rewrites BRAIN.md silently
• Users want to see what changed without manually comparing
• Builds trust in the auto-learning system ("what did it detect?")

Proposed Features

• Before/after diff displayed after update completes
• Categorized changes: "New repos detected", "Expertise level changes", "Velocity shifts"
• Colored terminal output (green=added, red=removed, yellow=changed)
• Optional --quiet flag to suppress diff
• History file (.engineer-brain/history/) to track BRAIN.md versions over time
• engineer-brain diff [days-ago] command to compare current vs. past state

Example Output

## Brain Updated — 2026-07-13

Changes detected:
+ New repo contributed to: ansible-navigator (3 commits)
~ Expertise change: Python moved from "Growing" → "Strong" (hit 10-commit threshold)
~ Velocity: 18 commits/week → 22 commits/week (+22%)
- Cooling repo: frontend-dashboard (no commits in 35 days)

Detailed diff:
  ## Active Repositories
+ | ansible-navigator | Contributor | Light | 2026-07-12 | CLI tooling |
  
  ## Expertise Map
  ### Strong
- - Python: 8 commits, growing
+ - Python: 11 commits, daily usage across 3 repos