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

.github/labeler.yml

@@ -6,7 +6,6 @@
 "channel: discord":
   - changed-files:
       - any-glob-to-any-file:
-          - "src/discord/**"
           - "extensions/discord/**"
           - "docs/channels/discord.md"
 "channel: irc":
@@ -28,7 +27,6 @@
 "channel: imessage":
   - changed-files:
       - any-glob-to-any-file:
-          - "src/imessage/**"
           - "extensions/imessage/**"
           - "docs/channels/imessage.md"
 "channel: line":
@@ -64,19 +62,16 @@
 "channel: signal":
   - changed-files:
       - any-glob-to-any-file:
-          - "src/signal/**"
           - "extensions/signal/**"
           - "docs/channels/signal.md"
 "channel: slack":
   - changed-files:
       - any-glob-to-any-file:
-          - "src/slack/**"
           - "extensions/slack/**"
           - "docs/channels/slack.md"
 "channel: telegram":
   - changed-files:
       - any-glob-to-any-file:
-          - "src/telegram/**"
           - "extensions/telegram/**"
           - "docs/channels/telegram.md"
 "channel: tlon":
@@ -96,7 +91,6 @@
 "channel: whatsapp-web":
   - changed-files:
       - any-glob-to-any-file:
-          - "src/web/**"
           - "extensions/whatsapp/**"
           - "docs/channels/whatsapp.md"
 "channel: zalo":