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-stats/SKILL.md

@@ -42,7 +42,7 @@ Also run a `search_memories` call with `query="project"`, `limit=1` to measure r
 
 ### Step 3: Display
 
-Print a compact dashboard:
+Print a compact dashboard with an ASCII histogram for category distribution:
 
 ```
 ## mem0 Stats
@@ -54,15 +54,15 @@ Print a compact dashboard:
 
 ### Project Lifetime (<project_id>)
   Total memories:    <N>
+
   By category:
-    Architecture Decisions: <N>
-    Anti-Patterns:          <N>
-    Task Learnings:         <N>
-    Coding Conventions:     <N>
-    User Preferences:       <N>
-    Tooling & Setup:        <N>
-    Session State:          <N>
-    Other:                  <N>
+    decision          ████████████████ 24
+    convention        ██████████░░░░░░ 15
+    anti_pattern      ████░░░░░░░░░░░░  6
+    task_learning     ███░░░░░░░░░░░░░  5
+    user_preference   ██░░░░░░░░░░░░░░  3
+    session_state     █░░░░░░░░░░░░░░░  2
+
   Oldest memory:     <date>
   Newest memory:     <date>
 
@@ -73,4 +73,10 @@ Print a compact dashboard:
   Branch:            <branch>
 ```
 
-Skip any section with zero data. Don't show empty categories.
+**Histogram rules:**
+- Max bar width: 16 characters. Scale all bars relative to the highest count.
+- Use `█` for filled and `░` for empty. Right-align the count number.
+- Sort categories by count descending. Omit categories with 0 memories.
+- If only 1-2 categories exist, still show the histogram — it provides visual context.
+
+Skip any section with zero data.