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>

scripts/publish-ownership-snapshot.sh

@@ -464,7 +464,9 @@ MANIFEST_PATH="$OUTPUT_DIR/ownership-snapshot-manifest.json"
 
 # A rebuild for the published as-of date (e.g. after a parser fix) lists the
 # builds it replaces, so clients still holding one update without --force.
-if [[ "$BUILT_AS_OF" == "$PUBLISHED_AS_OF" && "$PUBLISHED_SHA256" =~ ^[0-9a-fA-F]{64}$ ]]; then
+# Only when it keeps at least the published history: clients never downgrade.
+if [[ "$BUILT_AS_OF" == "$PUBLISHED_AS_OF" && "$PUBLISHED_SHA256" =~ ^[0-9a-fA-F]{64}$ ]] \
+    && [[ "$PUBLISHED_RELEASE_COUNT" =~ ^[0-9]+$ ]] && (( BUILT_RELEASE_COUNT >= PUBLISHED_RELEASE_COUNT )); then
     replaces_tmp="$PUBLISH_WORKDIR/manifest-replaces.json"
     jq --arg published "$PUBLISHED_SHA256" \
         --argjson previous "$(jq -c '.snapshot.replaces_sqlite_sha256 // []' <<< "$PUBLISHED_MANIFEST" 2>/dev/null || echo '[]')" '