feat: add channels directory with 44 channel adapter definitions (#21)

* feat: add channels directory with 44 channel adapter definitions

Add TOML definitions for all 44 channel adapters supported by LibreFang,
matching the adapters in librefang-channels crate source code.

Each channel file includes: id, name, description, category, icon,
protocol, and metadata (url, docs).

Categories: messaging, social, enterprise, developer, iot, email.

Also adds channels template at templates/channel.toml.

* fix: format templates/channel.toml with taplo

channels/discourse.toml

@@ -0,0 +1,10 @@
+id = "discourse"
+name = "Discourse"
+description = "Discourse forum adapter for posting topics and replies via the Discourse API"
+category = "developer"
+icon = "💬"
+protocol = "rest-api"
+
+[metadata]
+url = "https://www.discourse.org"
+docs = "https://docs.discourse.org"