refactor: remove channel shim directories, point all imports to extensions (#45967)

* 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

vitest.config.ts

@@ -183,16 +183,8 @@ export default defineConfig({
         "src/tui/**",
         "src/wizard/**",
         // Channel surfaces are largely integration-tested (or manually validated).
-        "src/discord/**",
-        "src/imessage/**",
-        "src/signal/**",
-        "src/slack/**",
         "src/browser/**",
         "src/channels/web/**",
-        "src/telegram/index.ts",
-        "src/telegram/proxy.ts",
-        "src/telegram/webhook-set.ts",
-        "src/telegram/**",
         "src/webchat/**",
         "src/gateway/server.ts",
         "src/gateway/client.ts",