mempalace-session: make --dry-run dedup-aware

A --dry-run report showed all qualifying sessions without indicating
which would actually hit the palace on a real run. On a second run
against an already-mined corpus this was misleading — output said
'Exported 62 session(s)' but the real mine step would skip all 62.

The wrapper now queries the palace's chroma.sqlite3 (read-only, via
file:...?mode=ro URI) for source_file values under the staging dir,
then tags each exported session as [NEW] or [SKIP] during listing and
reports the split in the summary:

  Exported 62 session(s) to ~/.cache/mempalace-session/wing_conversations
    0 new   → will be filed on mine
    62 already filed → will be skipped (dedup by source_file)

  --dry-run: no new sessions to mine. A real run would skip all 62.

Implementation notes:
- Classification is best-effort. If the palace is unreachable (fresh
  install, moved, permission-denied, file missing) the wrapper falls
  back to treating all exports as NEW — the real mine step still
  delegates dedup to 'mempalace mine --mode convos' which is the
  authoritative source of truth. Getting the classification wrong
  in --dry-run is cosmetic; behaviour of a real run is unchanged.
- Palace path respects $MEMPALACE_PATH env var for non-default setups.
- Same classification also shown on a real (non-dry-run) mine so users
  see upfront how much of the export set is actually new before the
  miner runs.

Verified both directions:
- All-already-filed case (current box, 62 sessions in palace): reports
  0 new, 62 skipped. --dry-run message correctly says 'would skip all'.
- Partial case (simulated by deleting one session's metadata from
  palace): reports 1 new, 61 skipped. --dry-run message correctly
  says 'would file 1 new'. Palace was restored from backup
  immediately after the test.

README and SKILL.md both updated with the new dedup-aware output and
a direct answer to the FAQ 'will it mine the same sessions again?'

bin/mempalace-session

@@ -66,10 +66,19 @@ Options:
   --agent <name>         Agent name recorded on drawers (default: $USER)
   --db <path>            Path to opencode.db (default: $OPENCODE_DB or
                          ~/.local/share/opencode/opencode.db)
-  --dry-run              Export + list; do not mine into palace
+  --dry-run              Export + list; do not mine into palace. Each session
+                         is tagged [NEW] or [SKIP] based on whether its
+                         source_file is already present in the palace.
   --no-repair            Skip `mempalace repair` after mining
   -h, --help             Show this help
 
+Idempotency:
+  Re-running on the same corpus is safe. The export step always writes every
+  qualifying session to the cache, but the mine step dedups on source_file
+  path — already-filed sessions are skipped without re-embedding. A --dry-run
+  summary shows exactly how many of the exported files are new vs already
+  filed, so you can see in advance what a real run would do.
+
 What gets mined:
   - Each qualifying session → one Claude Code JSONL file
   - Staged under ~/.cache/mempalace-session/<wing>/
@@ -139,6 +148,12 @@ mkdir -p "$STAGE"
 # ── Export sessions (Python heredoc) ────────────────────────────────
 # Writes one JSONL file per qualifying session into $STAGE.
 # Prints: EXPORTED <count>  on stdout, plus per-session lines.
+#
+# If the palace is reachable, also classifies each export as NEW or ALREADY
+# FILED (matching by source_file path) so --dry-run can report the true
+# mine-set size, not just the export-set size. Classification is advisory
+# only — the real mine step delegates dedup to `mempalace mine --mode convos`,
+# which is the authoritative source of truth.
 export_count=$(python3 - "$OPENCODE_DB" "$STAGE" "$SESSION_ID" "$SINCE" "$MIN_MESSAGES" <<'PY'
 import sqlite3, json, sys, os
 from datetime import datetime, timezone
@@ -157,6 +172,28 @@ if since:
         print(f"error: --since must be YYYY-MM-DD, got {since!r}", file=sys.stderr)
         sys.exit(1)
 
+# ── Load palace's already-filed source_files (best-effort, read-only) ──
+# Key the dedup check on absolute staging path. The palace stores these in
+# chroma.sqlite3 under embedding_metadata.key='source_file'. If the palace
+# isn't reachable (first install, moved, permission-denied), we fall through
+# to "everything is new" — the mine step will do the real dedup anyway.
+already_filed = set()
+palace_path = os.environ.get("MEMPALACE_PATH", os.path.expanduser("~/.mempalace/palace"))
+chroma_db = Path(palace_path) / "chroma.sqlite3"
+if chroma_db.is_file():
+    try:
+        pcon = sqlite3.connect(f"file:{chroma_db}?mode=ro", uri=True)
+        for (sf,) in pcon.execute(
+            "SELECT DISTINCT string_value FROM embedding_metadata "
+            "WHERE key='source_file' AND string_value LIKE ?",
+            (f"{stage}%",),
+        ):
+            if sf:
+                already_filed.add(sf)
+        pcon.close()
+    except sqlite3.Error:
+        pass  # palace unreachable → treat all exports as new (miner will dedup)
+
 conn = sqlite3.connect(f"file:{db_path}?mode=ro", uri=True)
 conn.row_factory = sqlite3.Row
 cur = conn.cursor()
@@ -181,6 +218,7 @@ if not sessions:
 # Prefetch messages + parts for qualifying sessions
 exported = 0
 skipped_short = 0
+skipped_already_filed = 0
 for sess in sessions:
     sid = sess["id"]
     cur.execute("SELECT COUNT(*) FROM message WHERE session_id=?", (sid,))
@@ -303,19 +341,26 @@ for sess in sessions:
         pass
 
     exported += 1
-    print(f"  {out_path.name}  ({msg_count} msgs, {len(out_lines)} turns)",
+    is_filed = str(out_path) in already_filed
+    if is_filed:
+        skipped_already_filed += 1
+    status = "SKIP" if is_filed else "NEW "
+    print(f"  [{status}] {out_path.name}  ({msg_count} msgs, {len(out_lines)} turns)",
           file=sys.stderr)
 
 print(f"EXPORTED {exported}")
+print(f"ALREADY_FILED {skipped_already_filed}")
 if skipped_short:
     print(f"SKIPPED_SHORT {skipped_short}", file=sys.stderr)
 PY
 )
 
-# Parse count from stdout
-count="${export_count##*EXPORTED }"
-count="${count%%[!0-9]*}"
+# Parse counts from stdout
+count="$(printf '%s\n' "$export_count" | awk '/^EXPORTED / { print $2 }')"
 count="${count:-0}"
+already_filed="$(printf '%s\n' "$export_count" | awk '/^ALREADY_FILED / { print $2 }')"
+already_filed="${already_filed:-0}"
+to_file=$(( count - already_filed ))
 
 if [[ "$count" -eq 0 ]]; then
   echo "no sessions qualified for export"
@@ -324,9 +369,17 @@ fi
 
 echo ""
 echo "Exported $count session(s) to $STAGE"
+echo "  $to_file new   → will be filed on mine"
+echo "  $already_filed already filed → will be skipped (dedup by source_file)"
 
 if [[ $DRY_RUN -eq 1 ]]; then
-  echo "--dry-run: skipping mine step"
+  if [[ "$to_file" -eq 0 ]]; then
+    echo ""
+    echo "--dry-run: no new sessions to mine. A real run would skip all $count."
+  else
+    echo ""
+    echo "--dry-run: skipping mine step. A real run would file $to_file new session(s)."
+  fi
   exit 0
 fi