fix(pm): restore hardlink→copy fallback on clone failure

[elrrrrrrr]

Summary

• hardlink_clone::clone_dir used to try fs::hard_link first and fall back to copy_file_sync on any error. Commit 13c698e (feat(pm): support multi-workspace selection for run command #2770, the multi-workspace refactor) rewrote the loop into an if/else that propagates the hardlink error, so cross-device installs now hard-fail:

  ERROR Failed to clone ... Invalid cross-device link (os error 18)
  

  This reproduces reliably with ut i -g on any machine where $HOME/.cache/nm and /usr/local/lib/node_modules are on different filesystems (e.g. Jenkins pods, some container images).
• Restore the fallback: on any fs::hard_link failure (EXDEV, EPERM, EMLINK, …), emit one tracing::warn! for the pair that failed and flip force_copy = true so the rest of this clone goes through copy_file_sync. Once one file in a src/dst pair fails the others will fail the same way, so retrying hardlink is wasted work.
• Renamed the local use_copy → force_copy — the variable now means “we were forced off the hardlink path,” which is a more accurate name once the flag can flip at runtime.
• Pre-feat(pm): support multi-workspace selection for run command #2770 the fallback was silent. The warn! makes the degraded-path condition visible in logs, which is how we would have caught this regression sooner.

Out of scope: the macOS clonefile path also has no cross-device fallback. The user's report is Linux, and the syscall/failure modes differ — handle as a follow-up.

Test plan

• cargo fmt -p utoo-pm
• cargo clippy -p utoo-pm --all-targets -- -D warnings --no-deps
• cargo test -p utoo-pm util::cloner (16 passed; the fallback branch is unreachable on a single-fs test host, which is expected)
• Manual cross-device repro on Linux: mount a tmpfs, point cache at it, run ut i -g node-gyp. Expect success with one hardlink failed … falling back to copy for remaining files warning per package.
• CI (Linux + Windows runners will fully exercise the hardlink_clone module, which is cfg-gated off on the macOS dev host).

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request updates the file cloning logic in crates/pm/src/util/cloner.rs to implement a fallback mechanism from hardlinking to copying. When a hardlink operation fails—for instance, due to cross-device link errors—the system now logs a warning and automatically switches to copying for the current and all subsequent files in the batch. I have no feedback to provide.