vault.save() produces non-deterministic JSON due to HashMap ordering

## Context

`serde_json::to_vec(secrets)` serializes HashMap with randomized key order, causing spurious vault file churn.

## Solution

Replace `HashMap` with `BTreeMap` in `AgeVaultProvider` or sort keys before serialization.

**Epic:** #858 | **Effort:** S | **Crate:** zeph-core