json-contracts.md says "20 files" — should be "20 run bundles" and drop legacy max_log_files reference

[codeforester]

Problem

docs/json-contracts.md:72–73 reads:

bounds default-log retention to the most recent 20 files (or the explicit max_log_files setting).

This is wrong on two counts:

1. Unit is bundles, not files. The code at app.py uses RetentionPolicy(max_bundles=_JSON_DEFAULT_MAX_LOG_FILES). A bundle is a per-run directory containing the log file plus run.json metadata. The unit "files" is imprecise and misleading — 20 bundles ≠ 20 log files in directory trees that also contain metadata.

2. max_log_files is the legacy API. max_log_files on App.__init__ is a compatibility shim for consumers who have not yet migrated to RetentionPolicy. The modern API is App(retention=RetentionPolicy(...)). Pointing JSON-mode users at max_log_files steers them toward the deprecated path.

Fix

Replace with:

bounds default-log retention to the most recent 20 run bundles. Pass an explicit RetentionPolicy to the App to override the limit, the maximum age, or the total cache size.