Support age-based retention in managed garbage collector (--delete-older-than)

[schickling-assistant]

Problem

The managed garbage collector in determinate-nixd uses a disk-pressure-based strategy (keeping 5-20% free), which works well for preventing disk-full situations. However, there's no way to configure age-based retention — equivalent to nix-collect-garbage --delete-older-than 30d.

Use case

On NixOS, nix.gc supports both dates (schedule) and options = "--delete-older-than 30d" for predictable, time-based pruning of old generations. On nix-darwin with determinateNix.enable = true, nix.gc is unavailable (nix.enable is false), and there's no equivalent in the determinateNixd.garbageCollector config.

Age-based pruning is useful for:

• Keeping a predictable rollback window (e.g. "always keep last 30 days")
• Preventing unbounded accumulation of generations even when disk space is plentiful
• Matching the behavior users expect from nix.gc on NixOS

Proposal

Add an option to the managed GC config (e.g. in determinateNixd.garbageCollector) for age-based retention, something like:

determinateNix.determinateNixd.garbageCollector = {
  strategy = "automatic";
  deleteOlderThan = "30d"; # optional, prune generations older than this
};

This would complement the existing disk-pressure strategy rather than replace it.

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Created on behalf of @schickling by Claude Code