fix: honor CORTEX_HOME for trusted workspaces

[Bortlesboat]

Summary:

• Resolve the TUI trusted-workspace store through cortex_common::get_cortex_home() so CORTEX_HOME is honored.
• Use the same resolved path for trusted-workspace reads and writes.
• Add focused tests for path resolution, read behavior, and write behavior under CORTEX_HOME.

Tests:

• cargo test -p cortex-tui trusted_workspaces -- --nocapture
• cargo check -p cortex-tui
• rustfmt --edition 2024 --check src/cortex-tui/src/runner/app_runner/trusted_workspaces.rs
• git diff --check

Note: cargo fmt --check --package cortex-tui currently reports an unrelated existing format diff in src/cortex-tui/src/question.rs on origin/main.

[greptile-apps]

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