feat: add template stats and oversized template warnings (Phase 1 of #90)

[nmccready]

Summary

Phase 1 of #90 — Detection & Warnings for oversized CloudFormation templates.

What's new

• --stats CLI flag — reports template statistics to stderr after processing:
  • Resource count with % of CloudFormation limit (500)
  • Output count with % of CloudFormation limit (200)
  • Template size in bytes with % of limit (1MB)
  • Breakdown of resource types and counts
• 80% threshold warnings — automatically warns when any metric reaches 80% of its CloudFormation limit (400 resources, 160 outputs, ~800KB size)
• All stats/warnings go to stderr so they don't interfere with template output on stdout

Docs

• Added docs/PHASE5-TEMPLATE-STATS-AUTOSPLIT.md — full roadmap for issue Feature: Auto-detect and split oversized CloudFormation templates (multi-stack) #90
• Moved completed phase docs (1-3) to docs/completed/

Usage

# Process template and show stats on stderr
cfn-include template.yml --stats

# Stats don't break piping
cfn-include template.yml --stats > output.json

What's NOT included (future phases)

• Auto-split suggestions
• Multi-stack generation
• Dependency analysis

Refs #90