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/xlsx.rs

@@ -9,7 +9,7 @@
 
 use std::collections::HashMap;
 use std::fs;
-use std::io::{Cursor, Read};
+use std::io::Cursor;
 use std::path::Path;
 
 use chrono::{Duration, NaiveDate};
@@ -83,11 +83,7 @@ fn read_zip_entry(
             )));
         }
     };
-    let mut output = String::new();
-    file.read_to_string(&mut output).map_err(|e| {
-        IdxError::ParseError(format!("failed to decode ownership XLSX entry {name}: {e}"))
-    })?;
-    Ok(Some(output))
+    super::read_zip_entry_bounded(&mut file, &format!("ownership XLSX entry {name}")).map(Some)
 }
 
 fn first_worksheet_name(zip: &mut zip::ZipArchive<Cursor<&[u8]>>) -> Result<String, IdxError> {