Fix cache subcommand detection when cache directory exists

[zhangli091011]

Summary

• keep the hidden cache subcommand enabled when ./cache is a directory
• continue treating a regular file named cache as input for cat compatibility
• add a deterministic integration test that clears an artifact from a cache directory in the working directory

Fixes #1726.

PR #3743 previously proposed the same production fix but was closed unmerged by its author. This version uses a smaller, cross-platform regression test and follows the current changelog workflow.

Tests

• cargo fmt --all -- --check
• cargo test --test integration_tests cache_
• cargo test --test integration_tests can_print_file_named_cache -- --exact
• cargo check --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings

AI Assistance

This change was implemented with AI assistance. The issue state, repository policies, code, diff, and test results were reviewed by the submitter before submission.

[zhangli091011]

The failing cargo audit/all-jobs checks are unrelated to this change. This PR does not modify Cargo.toml or Cargo.lock; the checked-in base lockfile contains crossbeam-epoch 0.9.18 (RUSTSEC-2026-0204) and quick-xml 0.39.4 (RUSTSEC-2026-0194/0195). All functional, platform, MSRV, quality, license, documentation, and changelog checks pass. I have not mixed an unrelated dependency upgrade into this focused cache-subcommand fix; this should clear when the base lockfile is updated or the repository handles the audit result.