Add Slack signal connector (pull decisions from channels)

[gleizerowicz]

Add a signal connector that pulls engineering decisions from Slack channels.

Currently the Slack bot can push digests to Slack and respond to queries. This issue is about the reverse: pulling signal from Slack channels (architecture discussions, decision threads, etc.) into the Wayfind content store.

What's needed

• New connector in bin/connectors/slack-signal.js
• Channel selection and filtering
• Thread-aware message extraction
• Decision detection heuristics
• Test coverage

See bin/connectors/github.js for reference.