fix: validate startup config paths

[Bortlesboat]

Summary

• Resolve startup write checks through the same config env precedence as the config loader
• Validate CORTEX_CONFIG_DIR and custom CORTEX_CONFIG parent paths before startup continues
• Add regression tests for invalid override paths

Platform bounty: PlatformNetwork/bounty-challenge#49355

Tests

• cargo test -p cortex-cli --bin Cortex check_cortex_home_writable_rejects -- --nocapture
• cargo check -p cortex-cli
• cargo fmt --check --package cortex-cli
• git diff --check

[greptile-apps]

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