fix(gateway): allow reply-only queue command

[rayjun]

What does this PR do?

Allows a bare /queue command to enqueue reply context when it is sent as a reply to another message. The existing handler preserved reply_to_* fields after enqueueing, but its usage guard only allowed text or media payloads, so reply-only /queue returned Usage: /queue <prompt> before the queued event could be built.

Related Issue

Follow-up to #55960

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• gateway/run.py: treats reply context as a valid /queue payload even when there is no prompt text or media attachment.
• tests/gateway/test_queue_command.py: adds a regression test for reply-only /queue, asserting the queued event keeps the reply metadata.

How to Test

1. Start an active gateway session.
2. Reply to an existing message with /queue and no additional text.
3. Verify the message is queued instead of returning Usage: /queue <prompt>, and the follow-up turn retains the quoted message context.

Automated checks:

python -m pytest tests/gateway/test_queue_command.py -q
python -m ruff check gateway/run.py tests/gateway/test_queue_command.py

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: Ubuntu 24.04 / Linux

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

For New Skills

N/A

Screenshots / Logs

$ python -m pytest tests/gateway/test_queue_command.py -q
......                                                                   [100%]
6 passed in 0.37s

$ python -m ruff check gateway/run.py tests/gateway/test_queue_command.py
All checks passed!