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_convo_miner.py

@@ -75,3 +75,86 @@ def test_mine_convos_does_not_reprocess_empty_chunk_files(capsys):
         assert "Files skipped (already filed): 1" in out2
     finally:
         shutil.rmtree(tmpdir, ignore_errors=True)
+
+
+def test_mine_convos_rebuilds_stale_drawers_after_schema_bump(capsys):
+    """When stored drawers have an older normalize_version, the next mine
+    silently purges them and refiles — no manual erase required.
+
+    This is what makes the strip_noise upgrade apply to existing corpora:
+    users just run `mempalace mine` again and old noise-filled drawers get
+    replaced with clean ones."""
+    from mempalace.palace import NORMALIZE_VERSION
+
+    tmpdir = tempfile.mkdtemp()
+    try:
+        convo_path = Path(tmpdir) / "chat.txt"
+        convo_path.write_text(
+            "> What is memory?\nMemory is persistence.\n\n"
+            "> Why does it matter?\nIt enables continuity.\n\n"
+            "> How do we build it?\nWith structured storage.\n"
+        )
+        palace_path = os.path.join(tmpdir, "palace")
+
+        # First mine — stamps drawers with NORMALIZE_VERSION
+        mine_convos(tmpdir, palace_path, wing="test")
+        capsys.readouterr()
+
+        client = chromadb.PersistentClient(path=palace_path)
+        col = client.get_collection("mempalace_drawers")
+        resolved = str(Path(tmpdir).resolve() / "chat.txt")
+        first_pass = col.get(where={"source_file": resolved})
+        first_ids = set(first_pass["ids"])
+        assert first_ids, "first mine should produce drawers"
+        for meta in first_pass["metadatas"]:
+            assert meta.get("normalize_version") == NORMALIZE_VERSION
+
+        # Simulate pre-v2 drawers: rewrite metadata to an older version,
+        # and replace content with "noise" so we can see it get cleaned up.
+        stale_metas = []
+        for meta in first_pass["metadatas"]:
+            stale = dict(meta)
+            stale["normalize_version"] = 1
+            stale_metas.append(stale)
+        col.update(
+            ids=list(first_pass["ids"]),
+            documents=["STALE NOISE"] * len(first_pass["ids"]),
+            metadatas=stale_metas,
+        )
+        # Add an extra orphan drawer that should also be purged.
+        col.add(
+            ids=["orphan_drawer"],
+            documents=["OLD ORPHAN"],
+            metadatas=[
+                {
+                    "wing": "test",
+                    "room": "default",
+                    "source_file": resolved,
+                    "chunk_index": 999,
+                    "normalize_version": 1,
+                }
+            ],
+        )
+        del col, client
+
+        # Second mine — version gate should trigger rebuild
+        mine_convos(tmpdir, palace_path, wing="test")
+        out = capsys.readouterr().out
+        assert (
+            "Files skipped (already filed): 0" in out
+        ), "stale drawers should force a rebuild, not a skip"
+
+        client = chromadb.PersistentClient(path=palace_path)
+        col = client.get_collection("mempalace_drawers")
+        rebuilt = col.get(where={"source_file": resolved})
+        # Orphan is gone
+        assert "orphan_drawer" not in rebuilt["ids"]
+        # No stale content survived
+        assert all("STALE NOISE" not in d for d in rebuilt["documents"])
+        assert all("OLD ORPHAN" not in d for d in rebuilt["documents"])
+        # All rebuilt drawers carry the current version
+        for meta in rebuilt["metadatas"]:
+            assert meta.get("normalize_version") == NORMALIZE_VERSION
+        del col, client
+    finally:
+        shutil.rmtree(tmpdir, ignore_errors=True)