Add unit tests for config validation and defaults

[btwshivam]

Problem Statement

The internal/config/ package has a config_test.go file but needs more comprehensive test coverage for validation logic, default values, and environment variable overrides to meet the ≥80% coverage target.

Proposed Solution

Add table-driven unit tests covering:

1. Default values — verify all config defaults match the documented values in README (e.g., doctor duration = 30s, prometheus addr = :9090, log level = info)
2. Environment variable overrides — verify KERNO_LOG_LEVEL, KERNO_AI_API_KEY, KERNO_AI_PROVIDER are read correctly
3. Invalid config — verify validation catches invalid values (e.g., negative duration, unknown log level, invalid prometheus address)
4. Config file loading — verify YAML config file is parsed correctly

Relevant Files

• internal/config/config.go — the config struct and loading logic
• internal/config/config_test.go — existing tests to extend

Acceptance Criteria

• ≥80% test coverage on internal/config/ package
• Table-driven tests following Go conventions
• Tests pass with go test -race ./internal/config/...

Difficulty

Beginner — requires basic Go testing knowledge. Good way to learn the project's config system.