refactor: consumer-owned interfaces for dialogs, tools, and app services

[dgageot]

Several packages depended directly on concrete types from unrelated packages
just to call a handful of methods, which made those consumers harder to test
in isolation and tied their compile-time dependencies to implementation
details they didn't need. This change moves toward consumer-defined
interfaces: each package now declares the narrow contract it actually needs,
and the concrete types satisfy it implicitly.

Six areas are affected. The attention dialogs in pkg/tui/dialog no longer
depend on the app type directly. Five tool expanders
(pkg/tools/a2a, pkg/tools/builtin/openurl, pkg/tools/builtin/fetch,
pkg/tools/builtin/openapi, pkg/tools/builtin/webhook) each define their
own expansion contract instead of importing a shared concrete type. App
title generation in pkg/app now goes through a small interface rather than
a direct dependency. The RAG backend in pkg/tools/builtin/rag and the A2A
streaming sender are likewise behind interfaces owned by the tool package
that uses them. Finally, pkg/runtime narrows the contract it needs for
command resolution.

In every case the refactor is structural only: nil handling, evaluation
timing, and lifecycle ordering are unchanged, and shared domain types and
construction-time dependencies are still passed through as before. Each
change is paired with focused unit tests covering the new interface
boundary, and the full build, test, and lint suite is green for every
commit on the branch.