Phase 6b — Stacks as ACOs (new Linear ticket)

Phase 6b promotes stacks to first-class ACOs by adding full enrichment metadata columns (classification, key_entities, token_counts, relationships, content_hash, etc.) to the stacks table, along with a new `enrich_stack` tool that rolls up child card data, checks cache, and populates ACO frontmatter. It also updates `get_stack_contents` and `get_card_content` to return complete ACO fields in their responses and extends `capsule_from_content` with a `save_as: 'stack'` option for creating enriched stacks directly.