feat(mem0-plugin): add /mem0:stats, /mem0:health, /mem0:digest skills (Tier 6)

Observability tier:
- /mem0:stats: session + lifetime memory dashboard with category breakdown
- /mem0:health: 5-check diagnostic (API key, identity, MCP, write/read, tracker)
- /mem0:digest: weekly memory activity summary with time-scoped searches
- session_stats.py: add `peek` command (JSON without clearing), per-category counters

mem0-plugin/skills/mem0-digest/SKILL.md

@@ -0,0 +1,75 @@
+---
+name: mem0-digest
+description: >
+  Summarize recent memory activity for the current project. Shows new memories,
+  categories touched, and growth trends over the past 7 days.
+  TRIGGER: user runs /mem0:digest, or asks "weekly summary", "what's new in memory",
+  "mem0 digest", "memory recap".
+---
+
+# Mem0 Weekly Digest
+
+Summarize recent memory activity for the current project.
+
+## Execution
+
+### Step 1: Fetch recent memories
+
+Call `search_memories` in parallel with different time-scoped queries:
+
+1. `query="decisions made this week"`, `filters={"AND": [{"user_id": "<id>"}, {"app_id": "<pid>"}, {"created_at": {"gte": "<7 days ago YYYY-MM-DD>"}}]}`, `limit=20`
+2. `query="bugs errors fixes"`, same time filter, `limit=20`
+3. `query="patterns conventions learnings"`, same time filter, `limit=20`
+
+Also call `get_memories` with `user_id` + `app_id` to get the full count for comparison.
+
+### Step 2: Deduplicate and analyze
+
+Merge results by memory ID. For each memory, extract:
+- `created_at` date
+- `categories[0]` or `metadata.type`
+- First 100 chars of content
+
+Group into:
+- **New this week** (created in last 7 days)
+- **Older** (created before, but surfaced by search)
+
+Calculate:
+- Total memories in project
+- Memories added in last 7 days
+- Most active categories this week
+- Days with most activity
+
+### Step 3: Display
+
+```
+## mem0 Weekly Digest — <project_id>
+Period: <start_date> to <today>
+
+### New Memories This Week: <N>
+<category>: <count>
+  - <memory summary, 80 chars> (<date>)
+  - ...
+<category>: <count>
+  - ...
+
+### Activity Pattern
+Most active day: <day> (<N> memories)
+Categories touched: <list>
+
+### Project Totals
+Total memories: <N> (up <N> from last week)
+Top categories: <top 3 by count>
+
+### Highlights
+<2-3 sentence summary of the most important decisions, learnings, or patterns stored this week>
+```
+
+### Step 4: Empty state
+
+If no memories in the last 7 days:
+```
+No new memories in the past week for <project_id>.
+Total project memories: <N>.
+Tip: mem0 captures learnings automatically as you work. Start coding!
+```