refactor(runtime): simplify agent execution flow (#32)

* refactor(runtime): simplify agent execution flow

* refactor(fetch): route web research through webdiver

* fix(fetch): load embedded webdiver agent

* fix(config): qualify default deepseek model

* fix(test): expect qualified default model

* fix(runtime): use small tier for default model

* fix(runtime): keep model override tier-neutral

* refactor(fetch): reuse result renderer

* fix(fetch): accept exactly one prompt

* fix(fetch): support stdin prompt context

* fix(fetch): remove accidental fixture file

* docs(runtime): remove stale async references