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/README.md

@@ -0,0 +1,7 @@
+# Channel Adapters Registry
+
+This directory contains TOML metadata files for all supported LibreFang channel adapters.
+
+Each `.toml` file describes a single channel adapter including its name, description, category, protocol, and relevant links. These files are used by the LibreFang registry to discover and display available channel integrations.
+
+See `../templates/channel.toml` for the file format template.