Captures an important insight, decision, constraint, pattern, or discovery to persistent memory via the vault CLI — distinct from general note-taking in that entries are tagged, tiered, and retrievable across future sessions. Use when the user says "save this", "remember this", "note for later", "don't forget", or "keep this in mind", or when the conversation produces something worth preserving: architectural decisions, recurring bugs, hard-won constraints, naming conventions, or user preferences.