chore(skills): rename add-*-v2 → add-* and drop dead v1 channel skills

Renamed 13 skill folders to drop the -v2 suffix (the v2/v1 distinction
isn't load-bearing anymore — there is no v1 runtime). Deleted the four
v1 channel skills that occupied the rename target paths (add-discord,
add-slack, add-telegram, add-whatsapp); they targeted src/v1 which is
reference-only per CLAUDE.md.

Skill content still says "v2" in places — that's a follow-up commit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.claude/skills/add-slack/REMOVE.md

@@ -0,0 +1,6 @@
+# Remove Slack
+
+1. Comment out `import './slack.js'` in `src/channels/index.ts`
+2. Remove `SLACK_BOT_TOKEN` and `SLACK_SIGNING_SECRET` from `.env`
+3. `pnpm uninstall @chat-adapter/slack`
+4. Rebuild and restart