feat: agit diff — semantic diff between commits

[Madhurr]

What

Implements agit diff — compares agent reasoning between any two commits.

Shows what changed in intent, confidence, risks, unknowns, alternatives, and other metadata. Not the code diff — the reasoning diff.

Usage

agit diff              # HEAD~1 vs HEAD
agit diff abc1234      # abc1234 vs HEAD  
agit diff abc def      # any two commits
agit diff --json       # machine-readable
agit diff --files      # include file changes

Example output

agit diff 2f5c0ee..d73d771

12 change(s) in agent context

Changed:
  Intent: "Unblock binary releases..." → "Enable agents and humans to see how reasoning evolved..."
  Confidence: 99% → 95%
  Agent: "claude-code (claude-sonnet-4-6)" → "anton (claude-opus-4-6)"

Added:
  + New alternative rejected: Simple text diff of JSON notes
  + New risk: [low] merge-commits
  + New unknown: Whether GitHub App should show diff between PR commits

Resolved:
  ✓ Risk resolved: [medium] github-api
  ✓ Unknown resolved: GitHub API rate limits

New files

• internal/drift/drift.go — semantic diff engine
• internal/drift/drift_test.go — 14 tests
• cmd/diff.go — CLI command with colored output

Tests

All 14 drift tests + existing tests pass.

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Committed with agit by Anton 🐺 — check Agent Context comment below 👇