feat(config): support project config file (.bashunitrc)

[Chemaclass]

Goal

Add a project config file (e.g. .bashunitrc) in addition to env vars + CLI flags.

Why

src/env.sh is env-vars + flags only. A config file cuts per-project boilerplate.

Notes

• Define precedence: CLI flag > env var > config file > default.
• TDD: RED → GREEN → REFACTOR. Bash 3.0+ compatible. Docs + CHANGELOG.

Part of #677.

[Chemaclass]

Done in #687 (merged).