refactor(config-reader): cache base path resolution

[amitksingh1490]

Summary

Stabilize Forge config path behavior so the process uses a single consistent base directory decision for the full runtime.

Context

Config path selection influences where Forge reads and writes user settings. Recomputing that decision repeatedly in one process can introduce unnecessary variability and make behavior harder to reason about when environment state changes between calls. This change makes path resolution deterministic for the process lifetime while preserving current legacy/default fallback semantics.

Changes

• Cache the resolved config base path once per process and return that cached value from base_path().
• Keep resolver logic in a dedicated function so tests can continue validating path precedence behavior directly.
• Update base-path tests to assert resolver behavior explicitly.

Testing

• cargo build -p forge_main
• FORGE_LOG=debug ./target/debug/forge
• FORGE_LOG=debug ./target/debug/forge
• cargo +nightly fmt --all && cargo +nightly clippy --all-features --workspace --all-targets --fix --allow-dirty -- -D warnings