fix(router): honor stats filePath

[snvtac]

Summary

• preserve router.stats.filePath when parsing router stats config
• write router stats to the exact configured file path instead of always using stats.jsonl
• treat a blank filePath as unset so existing/default stats persistence still works

Root cause

parseStats accepted router.stats but did not carry filePath into RouterStatsConfig. Even if a caller provided filePath directly, TokenStatsCollector only used it to derive a directory and then always wrote stats.jsonl.

Impact

Users and tests can now configure a full stats output path such as /tmp/custom-name.jsonl and the collector will write to that exact file. Existing configs without filePath, or with an empty templated value, continue to use the default ~/.pilotdeck/router/stats.jsonl.

Validation

• PILOTDECK_SKIP_BOOTSTRAP=1 npm test
• git diff --check

Closes #251