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/tests/test_session_stats.py

@@ -189,6 +189,49 @@ def test_category_counts_empty_category_not_tracked(_isolate_stats_file):
     assert data["category_counts"] == {}
 
 
+def test_recent_ids_tracked(_isolate_stats_file):
+    """record_add with memory_id stores ID in recent_ids."""
+    import session_stats
+
+    session_stats.init()
+    session_stats.record_add("decision", "abc-123")
+    session_stats.record_add("convention", "def-456")
+
+    with open(_isolate_stats_file) as f:
+        data = json.load(f)
+    assert len(data["recent_ids"]) == 2
+    assert data["recent_ids"][0]["id"] == "abc-123"
+    assert data["recent_ids"][1]["id"] == "def-456"
+    assert data["recent_ids"][0]["category"] == "decision"
+
+
+def test_recent_ids_capped(_isolate_stats_file):
+    """recent_ids list is capped at MAX_RECENT_IDS."""
+    import session_stats
+
+    session_stats.init()
+    for i in range(60):
+        session_stats.record_add("test", f"id-{i}")
+
+    with open(_isolate_stats_file) as f:
+        data = json.load(f)
+    assert len(data["recent_ids"]) == session_stats.MAX_RECENT_IDS
+    assert data["recent_ids"][0]["id"] == f"id-{60 - session_stats.MAX_RECENT_IDS}"
+
+
+def test_recent_ids_empty_without_memory_id(_isolate_stats_file):
+    """record_add without memory_id doesn't add to recent_ids."""
+    import session_stats
+
+    session_stats.init()
+    session_stats.record_add("decision")
+    session_stats.record_add("convention", "")
+
+    with open(_isolate_stats_file) as f:
+        data = json.load(f)
+    assert data["recent_ids"] == []
+
+
 def test_cli_peek(tmp_path):
     """Test CLI invocation: session_stats.py peek outputs JSON."""
     env = {**os.environ, "USER": "test"}