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>

tests/agents/agent-smoke.sh

@@ -81,9 +81,10 @@ done
 work="$(mktemp -d "${TMPDIR:-/tmp}/idx-agent-smoke.XXXXXX")"
 cleanup() {
     if [[ "$keep" == 1 ]]; then
-        # Never leave copied agent credentials behind.
-        rm -f -- "$work/codex/auth.json" "$work/pi/auth.json" "$work/hermes/auth.json" "$work/hermes/.env"
-        echo "agent-smoke: kept $work (copied credentials removed)"
+        # Never leave copied agent logins or settings (which may hold API keys)
+        # behind; each run's raw output is already under runs/.
+        rm -rf -- "$work/codex" "$work/pi" "$work/hermes" "$work/opencode"
+        echo "agent-smoke: kept $work (copied agent configs removed)"
     elif [[ "$work" == */idx-agent-smoke.* ]]; then
         rm -rf -- "$work"
     fi