Extract a shared helper that builds AuthManager from Config and applies
the forced ChatGPT workspace override in one place.
Create the shared AuthManager at MessageProcessor call sites so that
upcoming new transport's initialization can reuse the same handle, and
keep only external auth refresher wiring inside `MessageProcessor`.
Remove the now-unused `AuthManager::shared_with_external_auth` helper.
This PR adds root and profile config switches to omit the generated
`<permissions instructions>` and `<apps_instructions>` prompt blocks
while keeping both enabled by default, and it gates both the initial
developer-context injection and later permissions diff injection so
turning the permissions block off stays effective across turn-context
overrides.
Also added a prompt debug tool that can be used as `codex debug
prompt-input "hello"` and dumps the constructed items list.
rhan-oai