feat(normalize): auto-rebuild stale drawers via NORMALIZE_VERSION schema gate

Without this, the strip_noise improvement only helps new mines. Every
user who had already mined Claude Code JSONL sessions would keep their
noise-polluted drawers forever, because convo_miner's file_already_mined
skip short-circuits before re-processing.

Adds a versioned schema gate so upgrades propagate silently:

- palace.NORMALIZE_VERSION=2 — bumped when the normalization pipeline
  changes shape (this PR's strip_noise is the v1→v2 bump).
- file_already_mined now returns False if the stored normalize_version
  is missing or less than current, triggering a rebuild on next mine.
- Both miners stamp drawers with the current normalize_version.
- convo_miner now purges stale drawers before inserting fresh chunks
  (mirrors miner.py's existing delete+insert), extracted into
  _file_convo_chunks helper to keep mine_convos under ruff's C901 limit.

User experience: upgrade mempalace, run `mempalace mine` as usual, old
noisy drawers get silently replaced with clean ones. No erase needed,
no "you need to rebuild" changelog footgun.

Tests:
- test_file_already_mined_returns_false_for_stale_normalize_version —
  pins the version gate contract for missing/v1/current.
- test_add_drawer_stamps_normalize_version — fresh project-miner drawers
  carry the field.
- test_mine_convos_rebuilds_stale_drawers_after_schema_bump — end-to-end
  proof that a pre-v2 palace gets silently cleaned on next mine, with
  orphan drawers purged and NOT skipped.

Existing test_file_already_mined_check_mtime updated to include the
new field; all other tests unaffected.

tests/test_miner.py

@@ -7,7 +7,7 @@ import chromadb
 import yaml
 
 from mempalace.miner import mine, scan_project, status
-from mempalace.palace import file_already_mined
+from mempalace.palace import NORMALIZE_VERSION, file_already_mined
 
 
 def write_file(path: Path, content: str):
@@ -227,11 +227,17 @@ def test_file_already_mined_check_mtime():
         assert file_already_mined(col, test_file) is False
         assert file_already_mined(col, test_file, check_mtime=True) is False
 
-        # Add it with mtime
+        # Add it with mtime + current normalize_version
         col.add(
             ids=["d1"],
             documents=["hello world"],
-            metadatas=[{"source_file": test_file, "source_mtime": str(mtime)}],
+            metadatas=[
+                {
+                    "source_file": test_file,
+                    "source_mtime": str(mtime),
+                    "normalize_version": NORMALIZE_VERSION,
+                }
+            ],
         )
 
         # Already mined (no mtime check)
@@ -253,7 +259,12 @@ def test_file_already_mined_check_mtime():
         col.add(
             ids=["d2"],
             documents=["other"],
-            metadatas=[{"source_file": "/fake/no_mtime.txt"}],
+            metadatas=[
+                {
+                    "source_file": "/fake/no_mtime.txt",
+                    "normalize_version": NORMALIZE_VERSION,
+                }
+            ],
         )
         assert file_already_mined(col, "/fake/no_mtime.txt", check_mtime=True) is False
     finally:
@@ -296,3 +307,78 @@ def test_status_missing_palace_does_not_create_empty_collection(tmp_path, capsys
     out = capsys.readouterr().out
     assert "No palace found" in out
     assert not palace_path.exists()
+
+
+# ── normalize_version schema gate ───────────────────────────────────────
+#
+# When the normalization pipeline changes shape (e.g., strip_noise lands),
+# `NORMALIZE_VERSION` is bumped so pre-existing drawers can be silently
+# rebuilt on the next mine. These tests pin that contract.
+
+
+def test_file_already_mined_returns_false_for_stale_normalize_version():
+    """Pre-v2 drawers (no field, or older integer) must not short-circuit."""
+    tmpdir = tempfile.mkdtemp()
+    try:
+        palace_path = os.path.join(tmpdir, "palace")
+        os.makedirs(palace_path)
+        client = chromadb.PersistentClient(path=palace_path)
+        col = client.get_or_create_collection("mempalace_drawers")
+
+        # Pre-v2 drawer: no normalize_version field at all
+        col.add(
+            ids=["d_old"],
+            documents=["old"],
+            metadatas=[{"source_file": "/fake/old.jsonl"}],
+        )
+        assert file_already_mined(col, "/fake/old.jsonl") is False
+
+        # Explicitly older version
+        col.add(
+            ids=["d_v1"],
+            documents=["v1"],
+            metadatas=[{"source_file": "/fake/v1.jsonl", "normalize_version": 1}],
+        )
+        assert file_already_mined(col, "/fake/v1.jsonl") is False
+
+        # Current version — short-circuits
+        col.add(
+            ids=["d_current"],
+            documents=["cur"],
+            metadatas=[
+                {
+                    "source_file": "/fake/current.jsonl",
+                    "normalize_version": NORMALIZE_VERSION,
+                }
+            ],
+        )
+        assert file_already_mined(col, "/fake/current.jsonl") is True
+    finally:
+        del col, client
+        shutil.rmtree(tmpdir, ignore_errors=True)
+
+
+def test_add_drawer_stamps_normalize_version(tmp_path):
+    """Fresh drawers carry the current schema version so future upgrades work."""
+    from mempalace.miner import add_drawer
+
+    palace_path = tmp_path / "palace"
+    palace_path.mkdir()
+    client = chromadb.PersistentClient(path=str(palace_path))
+    col = client.get_or_create_collection("mempalace_drawers")
+    try:
+        added = add_drawer(
+            collection=col,
+            wing="test",
+            room="notes",
+            content="hello",
+            source_file=str(tmp_path / "src.md"),
+            chunk_index=0,
+            agent="unit",
+        )
+        assert added is True
+        stored = col.get(limit=1)
+        meta = stored["metadatas"][0]
+        assert meta["normalize_version"] == NORMALIZE_VERSION
+    finally:
+        del col, client