feat(cache): implement comprehensive caching support

[jmgilman]

Summary

• Add multi-layer caching with 5 cache types: content, blocks, refs, manifests, indexes
• Implement per-cache enable/disable configuration
• Add cache commands: status, path, clear
• Add configurable ref cache TTL (default 5m)
• Add comprehensive integration tests for cache functionality
• Update README with caching documentation

Cache Types

Cache	Description
content	File content cache (deduplicated by hash across archives)
blocks	HTTP range block cache
refs	Tag to digest mappings
manifests	OCI manifest cache
indexes	Archive index cache

Configuration

cache:
  enabled: true
  dir: /custom/cache/path  # Optional
  ref_ttl: 5m              # TTL for refs cache

  # Per-cache control
  content:
    enabled: true
  indexes:
    enabled: false  # Example: disable specific cache

Test plan

• Unit tests pass (go test ./...)
• Integration tests pass (go test -tags=integration ./integration/...)
• CI passes (just ci)
• Cache integration test covers:
  • Initial empty state
  • Cache population via push/pull
  • Clear specific cache type
  • Clear all caches
  • Per-cache enable/disable via config
  • Custom cache directory
  • Invalid cache type error handling

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