Align uninstall log cleanup with actual logs directory

[DDDDDGCSM]

Summary

• make cortex logs resolve its directory via AppDirs instead of hand-building a cache path
• make cortex uninstall enumerate the same real logs directory used by cortex logs
• add focused regression tests covering cache-backed logs removal

Verification

• cargo check -p cortex-cli
• git diff --check

Notes

• I attempted targeted cargo test, but the workspace currently has an unrelated pre-existing error in src/cortex-cli/src/agent_cmd/tests.rs (read_file_with_encoding private import), so full test execution is not clean in this environment.

[greptile-apps]

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