session-memory Phase 0: ingest cursor + sweep entrypoint + config (T06)

- session_memory/core/cursor.py: size/mtime change detection sidecar
- session_memory/config.toml: store paths, retention caps, per-source
  globs (claude on, codex/grok off for Phase 1), repo->domain map
- session_memory/ingest.py: discover->normalize->store->digest->evict;
  --dry-run creates/writes nothing; python -m session_memory.ingest
- tests/test_ingest.py; live dry-run parsed 84/85 real local sessions

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

session_memory/core/cursor.py

@@ -0,0 +1,49 @@
+"""Per-source ingest cursors (design §6; T06).
+
+Tracks ``(path -> size, mtime)`` so sweeps re-ingest only changed/grown files.
+Persisted as a small JSON sidecar. Ingest itself is idempotent on
+``(session_uid, seq)`` in the store, so the cursor is an optimization, not a
+correctness requirement — a lost cursor just means a full (still-idempotent)
+re-scan.
+"""
+
+from __future__ import annotations
+
+import json
+import os
+from typing import Optional
+
+
+class Cursors:
+    def __init__(self, path: str):
+        self.path = path
+        self._data: dict[str, dict] = {}
+        if os.path.exists(path):
+            try:
+                with open(path, "r", encoding="utf-8") as f:
+                    self._data = json.load(f)
+            except (OSError, ValueError):
+                self._data = {}
+
+    def is_changed(self, file_path: str) -> bool:
+        """True if the file is new or has changed size/mtime since last seen."""
+        try:
+            stat = os.stat(file_path)
+        except OSError:
+            return False
+        prev = self._data.get(file_path)
+        return prev is None or prev.get("size") != stat.st_size or prev.get("mtime") != stat.st_mtime
+
+    def mark(self, file_path: str) -> None:
+        try:
+            stat = os.stat(file_path)
+        except OSError:
+            return
+        self._data[file_path] = {"size": stat.st_size, "mtime": stat.st_mtime}
+
+    def save(self) -> None:
+        os.makedirs(os.path.dirname(self.path) or ".", exist_ok=True)
+        tmp = self.path + ".tmp"
+        with open(tmp, "w", encoding="utf-8") as f:
+            json.dump(self._data, f)
+        os.replace(tmp, self.path)