fix(jq): return ExecResult::err instead of Error::Execution for stderr suppression

[chaliy]

Summary

• jq builtin returned Err(Error::Execution(...)) for parse/runtime errors
• This bypassed apply_redirections, so 2>/dev/null couldn't suppress errors
• Changed all error paths to return Ok(ExecResult::err(...)) with non-zero exit code
• Errors now flow through normal redirect handling

Test plan

• issue_277_jq_stderr_suppressed — jq '.foo' 2>/dev/null suppresses error
• All 41 existing jq unit tests pass (updated 5 that expected is_err())

Closes #277