fix: address final review (path traversal, sync races, MCP limits)

Findings from a Codex (gpt-6.1-sol, xhigh) review of v0.2.4..HEAD, each
verified in the code before fixing:

- high: ticker symbols reached cache file paths unchecked, so a symbol
  like `../../REPORT` read (and deleted, if unparseable) .json files
  outside the cache, reachable through MCP get_quotes. resolve_symbol now
  accepts only ticker characters, and cache keys are encoded as single
  safe path components.
- high: sync decided before downloading and replaced the database
  without rechecking; an import that landed meanwhile was erased. It now
  rechecks the local state before installing, and idx-mcp runs
  ownership_sync exclusively with ownership reads.
- replaces_sqlite_sha256 never downgrades history: the client requires
  the rebuild to keep at least the local release count, and the
  publisher lists a replaced build only then.
- cache writes are atomic (temp file + rename), so concurrent readers
  never see, and delete, a partial entry.
- idx-mcp caps symbols (20, deduplicated) and limits, and runs at most 4
  tool calls at once.
- each Yahoo client gets its own cookie jar, paired with its crumb.
- ZIP entries (XLSX, KSEI archives) are read with a 256 MiB bound;
  non-ASCII archive dates and huge percentages are parse errors instead
  of panics or overflow.
- agent-smoke --keep deletes every copied agent config, not only auth
  files.

CLI goldens unchanged; the MCP tool golden changes only to state the
new caps.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/idx-core/src/ownership/snapshot.rs

@@ -202,6 +202,14 @@ pub fn sync_snapshot(
 
     let install_result = (|| -> Result<(), IdxError> {
         validate_snapshot_db(&temp_path, &manifest.snapshot)?;
+        // The decision was made before the download; an import or edit that
+        // landed since must not be overwritten without --force.
+        if !force && !same_local_state(local_state.as_ref(), inspect_local_db(db_path)?.as_ref()) {
+            return Err(IdxError::DatabaseError(
+                "the local ownership database changed during sync; rerun `idx ownership sync`"
+                    .to_string(),
+            ));
+        }
         install_snapshot_file(&temp_path, db_path)
     })();
 
@@ -362,6 +370,21 @@ fn inspect_local_db(db_path: &Path) -> Result<Option<LocalSnapshotState>, IdxErr
     }))
 }
 
+fn same_local_state(
+    before: Option<&LocalSnapshotState>,
+    after: Option<&LocalSnapshotState>,
+) -> bool {
+    match (before, after) {
+        (None, None) => true,
+        (Some(before), Some(after)) => {
+            before.file_sha256.is_some()
+                && before.file_sha256 == after.file_sha256
+                && before.release_count == after.release_count
+        }
+        _ => false,
+    }
+}
+
 /// Moves committed WAL frames into the database file, so its hash covers local
 /// edits too. Returns false when changes remain in the WAL.
 fn checkpoint_wal(db_path: &Path) -> bool {
@@ -446,10 +469,12 @@ fn build_sync_decision(
         };
     }
 
-    if snapshot
-        .replaces_sqlite_sha256
-        .iter()
-        .any(|sha256| file_sha256.is_some_and(|local| sha256.eq_ignore_ascii_case(local)))
+    // A rebuild never silently drops history the local database has.
+    if snapshot.release_count >= local.release_count
+        && snapshot
+            .replaces_sqlite_sha256
+            .iter()
+            .any(|sha256| file_sha256.is_some_and(|local| sha256.eq_ignore_ascii_case(local)))
     {
         return SyncDecision {
             action: OwnershipSyncAction::Updated,
@@ -909,6 +934,20 @@ mod tests {
         assert!(!decision.should_download);
     }
 
+    #[test]
+    fn build_sync_decision_never_drops_history_through_a_rebuild() {
+        // A forced same-date rebuild with fewer releases lists the old build;
+        // a client holding it keeps its history instead of downgrading.
+        let earlier = "e".repeat(64);
+        let mut snapshot = same_date_snapshot(vec![earlier.clone()]);
+        snapshot.release_count = 1;
+        let mut local = same_release_local(&snapshot, &earlier);
+        local.release_count = 3;
+
+        let decision = build_sync_decision(Some(&local), &snapshot, false);
+        assert!(!decision.should_download, "{}", decision.reason);
+    }
+
     #[test]
     fn build_sync_decision_keeps_a_locally_modified_database() {
         // Same release metadata but a file matching no published build (local