Add Webex Channel

Adds Cisco Webex support via the Chat SDK bridge. NanoClaw doesn't ship channels in trunk — this skill copies the Webex adapter in from the channels branch.

The mechanical steps under Apply carry nc: directive fences: an agent reads the prose and applies them, and a parser can apply them deterministically from the same document. Every directive is idempotent, so the whole skill is safe to re-run; anything a parser can't apply falls back to the prose beside it.

Apply

1. Copy the adapter and its registration test

Fetch the channels branch and copy the Webex adapter and its registration test into src/channels/ (overwrite — the branch is canonical):