fix(core): avoid phantom project directory on config load

[phernandez]

Why

Loading an existing config currently constructs a throwaway BasicMemoryConfig() just to parse environment overrides. With no BASIC_MEMORY_HOME, that temporary model seeds and creates ~/basic-memory even when the configured project lives elsewhere.

This is an independent maintainer implementation for #1029 because contributor PR #1030 cannot pass the CLA gate.

What changed

• remove the throwaway environment-only config construction
• drop file-backed fields that have explicit environment overrides, allowing BaseSettings to parse those environment values directly
• add a regression proving an existing custom project config does not create the unused default directory
• prove scalar and JSON environment overrides still take precedence

Verification

• just fast-check
• uv run pytest tests/test_config.py tests/test_config_load_side_effects.py -q — 136 passed

Fixes #1029
Supersedes #1030

[phernandez]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

Reviewed commit: aeb66ff71d

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