Phase 6a — Stacklist MCP Enrichment Core (LAB-212)

Phase 6a of the Stacklist MCP project ships five enrichment tools (`enrich_content`, `enrich_url`, `enrich_batch`, `capsule_from_content`, `capsule_from_url`) that transform raw text or external URLs into Atomic Content Object (ACO) frontmatter, optionally saving capsules to a user's private Stacklist vault with a content-hash-based cache to skip redundant LLM calls. The spec details the full architecture flow, file map across `stacklist-mcp` and `acp-sdk` repos, eight ordered implementation steps gated by validation, locked design decisions (e.g., `sourceType='manual'`, batch max of 25, cache writes only on save), and a verification plan covering unit tests with mocked dependencies and integration tests against a real Supabase database.