fix(cli): reset SIGPIPE to default so piping to head/tail exits quietly

[platinummonkey]

Summary

• Piping pup output into head/tail (e.g. pup logs search --query='*' | head -n 20) panicked with failed printing to stdout: Broken pipe (os error 32). Rust ignores SIGPIPE by default, so a closed pipe surfaces as an EPIPE error, which println!/print! turn into a panic instead of a clean exit.
• Reset SIGPIPE to SIG_DFL at process start — the standard fix used by ripgrep, fd, and other Unix CLI tools — so the process now exits quietly via the signal instead of panicking.

Changes

• src/main.rs: add reset_sigpipe(), called at the top of main() (native target), plus a unit test (reset_sigpipe_tests::sets_default_disposition)
• Cargo.toml/Cargo.lock: add libc as a cfg(unix)-only target dependency (already a transitive dependency at the pinned version, no new crate introduced)

Testing

• Built both the pre-fix and post-fix binaries and ran pup completions bash | head -n 1:
  • Before: panics with thread 'main' panicked ... failed to write completion file: Broken pipe, exit code 101
  • After: exits cleanly via SIGPIPE, exit code 141
• cargo test reset_sigpipe passes
• cargo fmt --check passes
• cargo clippy --all-targets -- -D warnings passes

Related Issues

Fixes the broken pipe panic reported when piping pup output to head.

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