feat: add --exit-status / -e flag (exit code 1 for false/null/empty output)

[sridhar-3009]

Summary

Closes #1799.

Adds -e / --exit-status to mq, mirroring jq's --exit-status behaviour:

Output	Exit code
At least one truthy value	0
Only false or null values	1
No output	1

Everything else — empty string "", 0, [], {} — is truthy and exits 0.

Usage

# Exits 1 — heading not found
mq -e 'select(.type == "heading" and .depth == 99)' doc.md

# Exits 0 — found something
mq -e 'select(.type == "heading")' doc.md

# Useful in shell scripts
if mq -e 'select(.type == "code")' README.md > /dev/null; then
  echo "has code blocks"
fi

Implementation

• OutputArgs.exit_status: bool — new clap flag (-e / --exit-status)
• is_falsy() helper — matches RuntimeValue::None | Boolean(false)
• thread_local! { HAD_TRUTHY_OUTPUT } — tracks across print() calls without refactoring the streaming output architecture
• After process_batch / process_streaming, calls std::process::exit(1) when the flag is set and no truthy value was seen

[harehare]

@sridhar-3009
Thanks for the PR!

It looks like some tests are failing in CI. Could you take a look when you get a chance?

Let me know if you need any help!