Add a cron-style history-pruning policy helper (`prune_policy`)

[usmanimamu17-create]

Problem Statement. prune_history(keep_latest) and
prune_history_by_age(min_age_seconds) exist. Operators have no
politics for "prune on Sunday 02:00 UTC". The prune_policy would
embed higher-level retention rules in storage.

Expected Outcome.

• New struct PrunePolicy storing {keep_latest: u32, max_age_seconds: u64, cron_expr: String}.
• New functions: set_prune_policy, get_prune_policy, apply_prune_policy.
• The cron expression is informational; actual cadence is cron-driven externally.

Files: lib.rs, new tests.

[Elizabeth-dev270]

Allow me work on this sir maintainer

[grantfox-oss]

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Next steps:

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