🛡️ Sentinel: [HIGH] Fix session tokens persisted in localStorage

[dttdrv]

🚨 Severity: HIGH
💡 Vulnerability: The sessionStore fallback in src/core/storage/local.ts defaulted to localStorage when sessionStorage was unavailable. This caused sensitive session tokens (access_token, refresh_token) to be persisted to disk.
🎯 Impact: Any XSS or malicious extension could steal tokens and impersonate the user from persistent disk storage, increasing the risk of exfiltration via physical access.
🔧 Fix: Replaced the localStorage fallback with an isolated InMemoryStorage instance that implements the Storage interface.
✅ Verification: Ran npx vitest run and verified that no regressions were introduced. Confirmed tests passed and new logic is in place. Added a sentinel journal entry.

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PR created automatically by Jules for task 3966926279880142809 started by @dttdrv

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