feat(slack): add allowed_channels whitelist config

[CashWilliams]

What does this PR do?

Just like #7044 but for Slack.

Add SLACK_ALLOWED_CHANNELS (env var) / slack.allowed_channels (config.yaml) support to restrict the Slack bot to only respond in specified channels.

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

modified:   gateway/config.py
modified:   gateway/platforms/slack.py
modified:   hermes_cli/config.py
modified:   tests/gateway/test_slack_mention.py

How to Test

1. Add agent to Slack
2. Ensure agent can respond in any channel
3. Add a channel ID to SLACK_ALLOWED_CHANNELS
4. Ensure agent can no longer respond other channels

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:

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

[teknium1]

Merged via #21251 — your implementation was cherry-picked onto current main with authorship preserved in git log, and the same whitelist pattern was extended to Telegram, Mattermost, Matrix, and DingTalk on top. Thanks for the contribution!

#21251