[BUG] Database initialization fails when /var/lib/cortex/ exists but not writable & Ollama provider choice not persisted

[sujay-d07]

What happened?

Issue 1: Database Initialization Failure

When running cortex install without sudo, the application crashes with:

ERROR:cortex.installation_history:Failed to initialize database: unable to open database file
❌ Unexpected error: unable to open database file

Root Cause:
The _ensure_db_directory() method in InstallationHistory and SemanticCache classes only checks if the directory /var/lib/cortex/ can be created, but not if the user can actually write files inside it.
When /var/lib/cortex/ already exists (created by a previous sudo operation), mkdir(..., exist_ok=True) succeeds silently without raising PermissionError. The fallback to ~/.cortex/ never triggers, and SQLite fails when trying to create the database file.

Issue 2: Ollama Provider Choice Not Persisted

When a user selects Ollama as their provider (option 3), the choice is not saved. Every subsequent cortex command prompts the user to select a provider again.
Root Cause:
The prompt_for_key() method returns immediately for Ollama without offering to save the preference, unlike the behavior for API keys.

How to reproduce

Issue 1: Database Error

1. Ensure /var/lib/cortex/ exists and is owned by root:
   sudo mkdir -p /var/lib/cortex ls -la /var/lib/cortex/ # Should show: drwxr-xr-x root root
2. Run cortex as a non-root user:
   cortex install nginx
3. Select Ollama (option 3) when prompted for provider then it will crash with "unable to open database file"

Issue 2: Ollama Provider Not Saved

1. Remove any existing config:
   rm -f ~/.cortex/.env
2. Run cortex and select Ollama:
   cortex install nginx --dry-run # Select option 3 (Ollama)
3. Run cortex again:
   cortex install docker --dry-run
   This will remember Ollama preference, no prompt shown

Environment

• OS: Ubuntu 22.04+ / Debian 12+ (or any Linux distribution)
• Python: 3.10+
• User: Non-root user
• Condition: /var/lib/cortex/ directory exists (created by previous root/sudo operation)
• Ollama: Installed and running locally (for provider persistence issue)

Error output