feat: Externalize configuration with sensible defaults

[BETAER-08]

Problem

Paths and ignore patterns are hardcoded. Users need a way to override these without changing source code.

Proposed Solution

• Create src/core/config.rs.
• Implement a config loader that looks for amdb.toml in the project root.
• If amdb.toml is missing, fall back to default hardcoded values (e.g., .database, default ignore list).
• Allow overriding specific keys via Environment Variables (e.g., AMDB_DB_PATH).

Definition of Done

• Config struct and loader implemented.
• Defaults match current hardcoded values.
• amdb.toml can override ignore_patterns and db_path.