Add KAI_DB_PATH environment variable to override database location

[jschatz1]

Problem

The database path is hardcoded to `.kai/db.sqlite` via the `dbFile` const at `kai-cli/cmd/kai/main.go:58`. Users cannot override this without code changes.

Expected Behavior

Support a `KAI_DB_PATH` environment variable that overrides the default database location. Fall back to `.kai/db.sqlite` when unset.

Files to Modify

• `kai-cli/cmd/kai/main.go` — read `KAI_DB_PATH` from env with current value as default