refactor(R2d.161): carve internal/api/aiwatchnl subpackage

Move the watch-face NL response handler and tests into the aiwatchnl subpackage.

Rename the exported handler to Handler, wire router.go through aiwatchnl.NewHandler, and keep the aiWatchFaceNLResponseHandler router variable plus AIHandlers.WatchFaceNLResponse field intact.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: atulmgupta

internal/api/aiwatchnl/doc.go

@@ -0,0 +1,6 @@
+// Package aiwatchnl serves the AI watch-face natural-language response route
+// and production tool adapters used by the /api/v1/ai/watch/respond surface.
+// The package narrates read-only watch context without mutating fleet state.
+//
+// Layer: handler
+package aiwatchnl