Move all Signal channel implementation files from src/signal/ to
extensions/signal/src/ and replace originals with re-export shims.
This continues the channel plugin migration pattern used by other
extensions, keeping backward compatibility via shims while the real
code lives in the extension.
- Copy 32 .ts files (source + tests) to extensions/signal/src/
- Transform all relative import paths for the new location
- Create 2-line re-export shims in src/signal/ for each moved file
- Preserve existing extension files (channel.ts, runtime.ts, etc.)
- Change tsconfig.plugin-sdk.dts.json rootDir from "src" to "."
to support cross-boundary re-exports from extensions/
