[codex] Honor --vault in doctor checks

[rushillshah]

Scope

• Fixes doctor should honor --vault path #21.
• Routes doctor and setup-check through the existing vault argument resolver so --vault <path> is honored.
• Adds a regression test that initializes two vaults and verifies doctor --vault reports the requested target vault.

Validation

• npm run check
• node --test tests/cli-init.test.mjs
• npm test
• Targeted smoke: initialized two temp vaults, ran doctor --vault <target> from the other vault cwd, and verified the reported default vault matched the target.

Risk

Low. This only changes vault resolution for setup-check commands and reuses the existing getVaultFromArgs helper used by other vault-aware commands.

[rushillshah]

Maintenance reconciliation note (2026-07-10): CI is passing, the branch is mergeable, and there are no unresolved review threads, but this PR is still draft and review-required. Next action: mark ready/request review before merge.