Cache grows indefinitely — no eviction of expired entries

[silicon-intern]

Problem

The SQLite cache (internal/cache/cache.go) stores entries with TTL metadata but never deletes expired rows. Over time, the cache database will grow unbounded.

Expected Behavior

Expired cache entries should be periodically cleaned up. A simple approach would be to run a DELETE FROM cache_entries WHERE created_at + ttl_seconds < ? query:

• On cache Open() (prune on startup)
• Or periodically during Set() calls (e.g., every Nth write)

Impact

For long-running automation or agents calling the CLI frequently, the cache.db file will accumulate stale rows indefinitely, consuming disk space unnecessarily.

Suggested Fix

Add a Prune() method to cache.Store and call it during Open() or on a probabilistic basis during Set().