fix: propagate rg exit code in rtk grep for CLI parity

[polaminggkub-debug]

Summary

• rtk grep returned exit 0 for all failure modes where rg returns non-zero
• Now propagates exit codes: 1 (no match), 2 (error — bad regex, missing file)
• Surfaces rg stderr on error so users see why it failed

Before / After

Scenario	Before	After	rg native
No match	exit 0	exit 1	exit 1
Bad regex ([)	exit 0	exit 2	exit 2
Missing file	exit 0	exit 2	exit 2
Match found	exit 0	exit 0	exit 0

Test plan

• rtk grep "zzz" file → exit 1 (no match)
• rtk grep "[" file → exit 2 + stderr error message
• rtk grep "alpha" /nonexistent → exit 2 + stderr error message
• rtk grep "alpha" file → exit 0 (match found, unchanged)
• cargo test grep_cmd — 8 tests pass
• cargo test --all — 412 pass, 0 fail

Fixes #162

🤖 Generated with Claude Code

[pszymkowiak]

LGTM — tested locally, all good.

• 412 tests pass, no conflicts with master
• 82/83 smoke tests pass (1 false positive: test script hardcodes expected test count)
• Exit codes verified manually:
  • Match found → exit 0 ✅
  • No match → exit 1 ✅ (was 0)
  • Bad regex → exit 2 + stderr ✅ (was 0)
  • Missing file → exit 2 + stderr ✅ (was 0)
• No regression on any other command (ls, git, gh, find, grep, cargo, curl, etc.)

Fixes #162. Ready to merge.