feat: add hook_context kwarg to save_plugin_config and get_plugin_config extensible calls

[Krashnicov]

Summary

Adds an optional hook_context: dict kwarg to both call_plugin_hook() call sites in save_plugin_config() and get_plugin_config() in helpers/plugins.py.

Motivation

Plugins intercepting these hooks have no way to distinguish the caller context (e.g. UI display vs. service layer vs. agent). Without this, plugins that need to behave differently per caller — for example, a secrets manager that masks values for the UI but resolves them for services — cannot do so without fragile inspect.stack() hacks.

hook_context is a free-form dict passed as an additional kwarg. Existing plugin hooks that do not declare it receive it via **kwargs and are completely unaffected — fully backwards compatible.

Example callers can pass: {"caller": "ui"}, {"caller": "service"}, {"caller": "agent"}

Change

Minimal — hook_context= added to two call_plugin_hook() invocations. No logic changes. No new dependencies. No existing behaviour altered.

Backwards Compatibility

Fully backwards compatible. Plugin hooks not declaring hook_context receive it silently via **kwargs.

[frdel]

That does not make much sense to me. It's placed in get_plugin_config and will always produce "api" this way.

[Krashnicov]

Fixed — hook_context is now assembled from a caller: CallerContext parameter (Literal["ui","agent","api"], default "api") on both get_plugin_config and save_plugin_config rather than hardcoded inside the function body. The default preserves full backwards compatibility — all existing call sites receive "api" implicitly without modification. Callers that need to distinguish context can pass the appropriate value explicitly.