feat: usage tracking + knowledge import script

- usage_tracker.py: daily token/call buckets per user (home/{user}/usage.json)
- Hook into local backend (OpenAI usage field) and Gemini API (usage_metadata)
- Claude/Gemini CLI backends produce no structured token data and are not tracked
- Fix CLAUDE.md stale tool count (27 → 39) and refresh tool list
- scripts/import_knowledge.py: walk markdown dirs, chunk by H2, call local LLM
  for summaries, create AE journal entries with path-derived tags; resumable via
  state file; --dry-run and --limit flags for safe testing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/usage_tracker.py

@@ -0,0 +1,75 @@
+"""
+API usage and token tracking.
+
+Writes daily buckets to home/{username}/usage.json:
+
+  {
+    "2026-05-01": {
+      "gemini_api/gemini-2.0-flash": {"calls": 3, "prompt_tokens": 8400, "completion_tokens": 520},
+      "local/llama3.2:latest":       {"calls": 2, "prompt_tokens": 1200, "completion_tokens": 310}
+    }
+  }
+
+Claude CLI and Gemini CLI backends produce no structured token data and are not tracked.
+"""
+
+import asyncio
+import json
+import logging
+from datetime import date
+from pathlib import Path
+
+from config import settings
+
+logger = logging.getLogger(__name__)
+
+_LOCK = asyncio.Lock()
+
+
+def _usage_path(username: str) -> Path:
+    return settings.home_root() / username / "usage.json"
+
+
+async def record(
+    username: str,
+    backend: str,
+    model_name: str,
+    prompt_tokens: int,
+    completion_tokens: int,
+) -> None:
+    """Append one call's token counts to the daily usage log for this user.
+
+    backend    — "gemini_api" | "local"
+    model_name — the exact model string (e.g. "gemini-2.0-flash", "llama3.2:latest")
+    """
+    path = _usage_path(username)
+    today = date.today().isoformat()
+    key = f"{backend}/{model_name}"
+
+    async with _LOCK:
+        try:
+            data: dict = json.loads(path.read_text()) if path.exists() else {}
+        except Exception:
+            data = {}
+
+        entry = data.setdefault(today, {}).setdefault(
+            key, {"calls": 0, "prompt_tokens": 0, "completion_tokens": 0}
+        )
+        entry["calls"] += 1
+        entry["prompt_tokens"] += prompt_tokens
+        entry["completion_tokens"] += completion_tokens
+
+        try:
+            path.parent.mkdir(parents=True, exist_ok=True)
+            path.write_text(json.dumps(data, indent=2))
+        except Exception as e:
+            logger.warning("Failed to write usage data to %s: %s", path, e)
+
+
+def read_usage(username: str) -> dict:
+    """Return the full usage dict for this user. Empty dict if no file yet."""
+    path = _usage_path(username)
+    try:
+        return json.loads(path.read_text()) if path.exists() else {}
+    except Exception:
+        return {}