Keep project memory small and current: structure and hygiene #291

Implement the concept in #290: terse summaries, memory weight, one level of submemories (bags), a deterministic hygiene score and write-time feedback, so memory stays small and current in every project without one-off cleanups.

Order of work follows the subtasks: score and summary limit first, then weight, bags and grouped recall, then write-time feedback and guidance.

Complete, paginated discovery stays in #219 and recall truncation reporting in #215; bag children must use the same continuation contract. Related: #77 (session-end suggestions), #214 (supersedes), #216 (freshness).

Acceptance

• Every subtask done and #290 updated from "planned" to the implemented behaviour.
• MCP tool count stays within its budget test (#282); new behaviour extends existing tools where possible.