fix(command): resolve relative PATH entries against cwd

[RSS1102]

Summary

• resolve relative PATH entries against the command cwd before binary lookup
• preserve PATH ordering, empty entries, and ~ expansion semantics
• defensively normalize the resolved binary path
• cover relative matches, fallback search, empty PATH entries, and tilde preservation

Problem

which::which_in searches relative PATH entries against the process cwd rather than the cwd supplied for the command. The npm-distributed CLI can therefore fail to resolve node when Vite+ git hooks prepend ./node_modules/.bin, even though the binary exists.

Validation

• cargo test -p vp_command (17 passed)
• cargo test -p vp_global_cli relative_path_entry (1 passed)
• cargo clippy -p vp_command --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• reproduced the failure with vite-plus@0.2.7 and a relative PATH entry, then verified the same command reaches vp check with the patched NAPI binding

Fixes #2326

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[fengmk2]

@RSS1102 Can you add a snapshot test to cover this bug fix change?