fix(secrets): warn when the vault beside config.toml disagrees with the home vault

[chinkan]

Follow-up to #157 (issue #156).

Problem

import_config_dir_vault copies secrets from <config_dir>/secrets/vault into the home vault, and names already in the home vault win. When the same name holds a different value in both vaults, the old value was ignored silently, so a user who re-ran the wizard in another folder had no hint why a secret they just entered was not used.

Change

• import_config_dir_vault now returns (copied, conflicts). conflicts is the sorted list of names present in both vaults with different values. Values are never returned or logged.
• main.rs logs one warn per name: Secret `<name>` differs between vaults: using the home vault value, ignoring the vault beside config.toml.
• Behaviour is unchanged: the home vault still wins, nothing is overwritten, the old vault is kept.

Tests

• config_dir_vault_secrets_are_copied_without_overwriting now asserts the differing name is reported on the first and second run, and that a name with the same value in both vaults is not reported.
• No-op cases assert (0, []).
• cargo fmt --check, cargo clippy --locked --all-targets -- -D warnings, cargo test --locked: 921 passed.

[chinkan]

TL ACCEPT at 96fcb49 (comment — same-account Approve blocked).

Returns (copied, conflicts) with sorted key names only; main warns names, home still wins, behaviour unchanged. Tests cover conflict on first/second run and no-op (0, []). Merge after Product GO + QA GO + CI (Analyze (rust) green; AI findings 402 non-block). No tag.

Note: PR base is 71d349f; merge onto current main 655e6b6 is fine (no conflict expected with #158 script-only).