fix(auth): bound macOS Keychain status probes

[Stevvven777]

Summary

• add an optional existence-only SecretStore.has() operation and use it for credential status probes
• make macOS status call /usr/bin/security find-generic-password without -w, so it never requests the stored password
• bound credential-helper subprocesses to five seconds, kill a timed-out helper, and expose the stable additive reasonCode: CREDENTIAL_STORE_TIMEOUT
• keep secret load/write/delete paths separate and preserve their existing behavior

Failure and compatibility semantics

• auth status remains read-only and does not retry credential-store operations
• timeout leaves both the Keychain item and local profile metadata unchanged
• existing status fields and exit behavior remain compatible; reasonCode and the optional store method are additive
• Windows and Linux backends retain their current lookup behavior while gaining the same bounded external-helper runner where applicable

Validation

• macOS arm64, Node.js v26.8.1
• npm run check
• npm test — 314/314 passed
• npm pack --dry-run --json — passed
• npm audit --omit=dev — 0 vulnerabilities
• real macOS host regression with synthetic profile metadata and a nonexistent synthetic Keychain account — exit 0 in 0.28 seconds, no password read
• timeout fixture runs a genuinely non-returning helper and verifies bounded CREDENTIAL_STORE_TIMEOUT status with no automatic retry

Closes #9

[Stevvven777]

GitHub Actions created CI run https://github.com/wormforce/sustech-cli/actions/runs/33626389782, but it completed as action_required without starting jobs because this is a fork PR. Maintainer approval is needed to run it. Local macOS arm64 gates passed: typecheck, 314/314 tests, prepack dry-run, production audit with 0 vulnerabilities, and a real metadata-only synthetic Keychain status probe returned in 0.28 s. I have not retriggered the workflow.

[pentaoa]

Reviewed the metadata-only macOS status path, helper-process timeout handling, and credential status error projection. Status uses the optional existence probe instead of retrieving a password; the timeout has a stable reason code and does not retry the operation. No blocking implementation issue found.

During verification, the new Linux fixture failed during an ordinary write because it applied a 50 ms timeout to every helper invocation. Pushed 1e39512 to this maintainer-editable branch: normal operations now use the production timeout, while only the deliberately hanging lookup uses 50 ms. The build and all 16 credential/keyring/dashboard tests pass, including the bounded hanging-helper test.

Ready to merge once the updated cross-platform CI checks pass.