fix: skip rewriting find/fd in pipes (#439)

[pszymkowiak]

Summary

• Skip rewriting find/fd when followed by a pipe (|)
• rtk find outputs a grouped format (files space-separated) incompatible with pipe consumers like xargs, grep, wc, sort which expect one path per line
• find without a pipe is still rewritten to rtk find (savings preserved)

Before

find . -name "*.rs" | xargs grep "fn run"
→ rtk find . -name "*.rs" | xargs grep "fn run"
→ xargs receives "73F 4D: ./ main.rs lib.rs..." → FAILS

After

find . -name "*.rs" | xargs grep "fn run"
→ find . -name "*.rs" | xargs grep "fn run"  (not rewritten)
→ works correctly

Tests

• test_rewrite_find_pipe_skipped — find with xargs not rewritten
• test_rewrite_find_pipe_xargs_wc — find with wc not rewritten
• test_rewrite_find_no_pipe_still_rewritten — find alone still rewritten

Fixes #439

[pszymkowiak]

Tested manually:

• rtk rewrite "find src -name *.rs | xargs grep fn" → exit 1 (not rewritten) ✅
• rtk rewrite "find src -name *.rs" → rtk find src -name *.rs (rewritten) ✅
• /usr/bin/find src -name "*.rs" | /usr/bin/xargs grep -l "fn run" → works correctly with native find ✅
• 903 tests pass, 3 new tests added