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-mcp/tests/golden/tools.json

@@ -43,7 +43,7 @@
       "$schema": "https://json-schema.org/draft/2020-12/schema",
       "properties": {
         "symbols": {
-          "description": "IDX tickers, e.g. [\"BBCA\", \"BBRI\"]; the `.JK` suffix is optional.",
+          "description": "IDX tickers, e.g. [\"BBCA\", \"BBRI\"]; the `.JK` suffix is optional. At most 20.",
           "items": {
             "type": "string"
           },
@@ -125,7 +125,7 @@
         },
         "limit": {
           "default": null,
-          "description": "Maximum rows returned (default 200); `total` reports the full count.",
+          "description": "Maximum rows returned (default 200, at most 2000); `total` reports the full count.",
           "format": "uint",
           "minimum": 0,
           "type": [
@@ -202,7 +202,7 @@
       "$schema": "https://json-schema.org/draft/2020-12/schema",
       "properties": {
         "symbols": {
-          "description": "IDX tickers, e.g. [\"BBCA\", \"BBRI\"]; the `.JK` suffix is optional.",
+          "description": "IDX tickers, e.g. [\"BBCA\", \"BBRI\"]; the `.JK` suffix is optional. At most 20.",
           "items": {
             "type": "string"
           },
@@ -278,7 +278,7 @@
         },
         "limit": {
           "default": null,
-          "description": "Maximum results (default 50).",
+          "description": "Maximum results (default 50, at most 200).",
           "format": "uint",
           "minimum": 0,
           "type": [
@@ -310,7 +310,7 @@
       "properties": {
         "limit": {
           "default": null,
-          "description": "Maximum results (default 20).",
+          "description": "Maximum results (default 20, at most 100).",
           "format": "uint",
           "minimum": 0,
           "type": [