Browser session reuse between vault_login calls

**Why:** Currently each vault_login opens a new Chrome tab. If the user is already logged in (from a previous vault_login), we waste time re-authenticating.

**Scope:**
- Check if already logged in (cookie/session check) before filling login form
- Reuse existing Chrome context/tab
- Integrate with vault_check for pre-login validation

**Priority:** Low — optimization, not blocking