Compute a deterministic memory hygiene score with findings #292

See #290, section 4. Compute a 0–100 score per project from structure and timestamps only, without an LLM, and always return the findings that name the items.

Work

• Start with the findings that need no new data: crowded top level, long summary, expired but not archived, old memories of type task, old session summaries, duplicate or strongly overlapping normalized titles.
• Leave extension points for stale bags, lopsided bags and too many critical memories; the weight and bag subtasks add them.
• Show score and findings in the project overview; add one line to memory_recall (Hygiene 58/100 • …).
• Thresholds as named constants; document them on #290.

Acceptance

• Unit tests per finding, including the boundary of each threshold.
• Findings and counts include only memories the reader may see; no private titles leak through recall or the overview.
• The score is stable: the same data always yields the same score and order of findings.