auth: improve doctor output for expired or missing sessions

[shrey16]

Goal

Make bharatcode doctor more useful when auth state is missing, expired, unreadable, or inconsistent.

Expected behavior

bharatcode doctor should distinguish at least:

• credentials file missing
• credentials file permissions are too broad
• access token expired but refresh token exists
• refresh token missing or invalid-looking
• BharatCode engine config missing or not writable

Acceptance

• Output is human-readable and does not print token values.
• Tests cover the new status cases with mocked credential/config files.
• Existing successful doctor output remains concise.