* refactor: remove channel shim directories, point all imports to extensions
Delete the 6 backward-compat shim directories (src/telegram, src/discord,
src/slack, src/signal, src/imessage, src/web) that were re-exporting from
extensions. Update all 112+ source files to import directly from
extensions/{channel}/src/ instead of through the shims.
Also:
- Move src/channels/telegram/ (allow-from, api) to extensions/telegram/src/
- Fix outbound adapters to use resolveOutboundSendDep (fixes 5 pre-existing TS errors)
- Update cross-extension imports (src/web/media.js → extensions/whatsapp/src/media.js)
- Update vitest, tsdown, knip, labeler, and script configs for new paths
- Update guard test allowlists for extension paths
After this, src/ has zero channel-specific implementation code — only the
generic plugin framework remains.
* fix: update raw-fetch guard allowlist line numbers after shim removal
* refactor: document direct extension channel imports
* test: mock transcript module in delivery helpers
