fix: rtk ls summary line inflates pipe count

[FlorianBruniaux]

Problem

rtk ls <dir> | wc -l returns a count inflated by 2 (blank line + summary line) because the 📊 summary is always written to stdout.

rtk ls src/ | wc -l   # returns N+2 (wrong)
ls src/ | wc -l        # returns N (correct)

This breaks any script that pipes rtk ls to count files or process entries.

Unix Convention

Metadata/summary goes to stderr or is suppressed when stdout is not a TTY. RTK already uses this pattern in gain.rs via std::io::IsTerminal.

ls itself does this — no summary when piped.

Proposed Fix

Suppress the 📊 summary line when stdout is piped (not a TTY). Show it only in interactive mode.

Changes to src/ls.rs

• Add use std::io::IsTerminal;
• Split compact_ls() return value into (entries: String, summary: String) tuple
• In run(): always print entries, only print summary when stdout().is_terminal()
• Track only what is actually sent to stdout for rtk gain accuracy

Verification

# Pipe mode — should NOT show summary
rtk ls src/ | wc -l

# TTY mode — should show summary
rtk ls src/