Add KAI_AI_MODEL and KAI_AI_TIMEOUT environment variables

[jschatz1]

Problem

The AI model is hardcoded to `claude-sonnet-4-20250514` (`kai-cli/internal/ai/client.go:33`) and the timeout is hardcoded to 60 seconds (line 35). Contributors and users cannot test against different models or adjust timeouts without code changes.

Expected Behavior

• `KAI_AI_MODEL` env var to override the default model
• `KAI_AI_TIMEOUT` env var to override the default timeout (in seconds)

Files to Modify

• `kai-cli/internal/ai/client.go` — read from env vars with current values as defaults