Use internal model context fragments for goal steering

[jif-oai]

Why

Goal steering is one form of runtime-owned model context, but the old <goal_context> wrapper made the contextual-fragment hiding path goal-specific. Using a source-labeled internal context fragment gives core and extensions a shared shape for hidden model steering while keeping those prompts out of visible turn history.

The change also keeps legacy <goal_context> messages recognized as hidden contextual input so existing stored history does not start rendering old goal-steering prompts as user-visible turn items.

What Changed

• Replaces GoalContext with InternalModelContextFragment plus a validated InternalContextSource.
• Renders goal steering as <codex_internal_context source="goal">...</codex_internal_context>.
• Updates core goal steering and ext/goal steering to inject the new internal-context fragment.
• Updates contextual-fragment, event-mapping, goal, and session tests for the new wrapper.

Test Coverage

• Adds coverage for detecting the new internal model context fragment.
• Preserves coverage for hiding legacy <goal_context> fragments.
• Verifies invalid internal context sources are rejected and arbitrary context tags are not hidden.
• Updates goal steering/session assertions to expect the new source="goal" wrapper.

[jif-oai]

Windows is a transient failure

[etraut-openai]

Code looks good

I also manually tested that the TUI properly renders the goal prompt.