Decide and document zero-day heartbeat threshold behavior

[theDakshJaitly]

Follow-up from Copilot review on PR #39.

Problem

heartbeat.staleDays, memoryCleanupDays, and dailyMemoryRetentionDays currently use positive-number validation, so 0 is rejected and the default is used instead.

That means config cannot express: consider stale as soon as older than today.

Decision needed

Choose one of these behaviors:

1. Allow 0 for day-based heartbeat thresholds while still rejecting negatives.
2. Keep requiring positive values, but document that 0 is intentionally unsupported.

Acceptance criteria

• The intended behavior is documented in README/config docs.
• Tests reflect the chosen behavior.
• If 0 is allowed, config parsing accepts >= 0 for day-based heartbeat thresholds only.

Source: #39 (comment)