fix(mem0-plugin): fix 3 critical + 5 important issues from review

Critical fixes:
- C1: Pin/dream update_memory now reads content first via get_memory
  before updating, preventing metadata-only calls that wipe content
- C2: SessionStart memory count uses GET /v3/memories/?page_size=1 for
  real count from pagination envelope instead of search limit=100
- C3: on_task_completed.sh uses ${MEM0_PROJECT_ID:-unknown} to avoid
  crash when identity resolution fails under set -u

Important fixes:
- I1: session_stats.py tracks recent_ids (memory IDs + timestamps) for
  /mem0:forget undo support. Capped at 50 entries.
- I2: /mem0:health --deep mode adds memory quality analysis: duplicate
  detection, stale entry scan, contradiction flags, orphan tagging
- I3: /mem0:stats ASCII histogram for category distribution with bar
  chart, sorted by count descending
- I4: mem0-mcp skill documents access counter pattern: get_memory then
  update_memory to track which memories are actually used
- I5: on_pre_commit.py reads COMMIT_EDITMSG first (current commit),
  falls back to HEAD only if EDITMSG unavailable

Tests: 91 -> 94 (3 new for recent_ids tracking)

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

@@ -59,3 +59,18 @@ Deleted <N> memories.
 ```
 
 If any deletions failed, report which ones and why.
+
+## Undo recent writes
+
+If the user says "undo last N memories" or "undo last write":
+
+1. Read session stats to get recently written memory IDs:
+   ```bash
+   SCRIPT_DIR="${CLAUDE_PLUGIN_ROOT:-${CODEX_PLUGIN_ROOT:-${CURSOR_PLUGIN_ROOT:-}}}/scripts"
+   python3 "$SCRIPT_DIR/session_stats.py" peek
+   ```
+2. Parse the `recent_ids` array from the JSON output. Each entry has `id`, `category`, `ts`.
+3. Show the last N entries (default 1) and ask for confirmation.
+4. Delete confirmed entries via `delete_memory`.
+
+If `recent_ids` is empty, tell the user: "No recent memory IDs tracked this session. Use a search query instead."