test: add comprehensive gh-pages behavioral snapshot tests

Add 25 behavioral snapshot tests for gh-pages v3.2.3 to detect any
behavioral changes when upgrading gh-pages versions. These tests
capture EXACT internal behavior of gh-pages.publish() by mocking
child_process and verifying all git commands executed.

Coverage:
- All 8 options we pass to gh-pages (repo, branch, message, remote,
  git, add, dotfiles, user)
- All 7 gh-pages defaults we rely on (depth, history, tag, push,
  dest, src, remove, silent)
- 3 error scenarios (clone failure/retry, no changes, no user)
- File copy operation with exact file verification
- Command execution order verification
- gh-pages.clean() actual filesystem deletion

Key features:
- Command whitelist to fail fast on unexpected git commands
- Deterministic mocking with currentTestContext
- Precise assertions (exact values, exact positions)
- Comprehensive documentation explaining WHY each test exists
- Tests only what angular-cli-ghpages ACTUALLY uses

Total tests: 340 (up from 315)
Behavioral tests: 25
All tests passing: ✓

Author: JohannesHoppe