fix(analyze): prune expired cache files (#902)

* fix(analyze): prune expired cache files

* fix(analyze): prune cache by scan time

* fix(analyze): revert to mtime-based pruning, always async

Reverts 647e12e. The gob-decode approach was unnecessary overhead:
pruning is opportunistic disk reclamation, not a correctness gate.
loadCacheFromDisk already enforces ScanTime-based expiry at read time,
so the pruner only needs to remove files old enough by wall-clock age.

Use mtime for the expiry check (fast, no I/O into file contents) and
run pruning as a background goroutine in both TUI and --json modes.