Let extensions contribute World State sections (#30100)

## Why

#29856 already owns the durable thread intent and exact environment
binding. This PR adds only the small missing extension boundary: an
extension can contribute one named World State section, while core still
owns persistence, diffing, and model-visible fragment types.

This lets skills stay in the skills extension instead of moving their
runtime into core.

## Shape

```text
extension-owned state
        |
        | contribute section id + JSON snapshot + renderer
        v
core World State
        |
        | compare with the previous snapshot
        v
no message, or one incremental model-visible update
```

The extension API is deliberately small:

```rust
fn contribute_world_state(...) -> Vec<WorldStateSectionContribution>
```

Core adapts the rendered result to `ContextualUserFragment`, records the
snapshot, and keeps the existing compaction/resume behavior.

## What changes

- Adds extension-owned World State section contributions.
- Calls those contributors from the existing per-step World State
builder.
- Restores durable selected capability roots into extension thread state
on resume.
- Keeps the actual model-context fragment and rollout machinery in core.

## What does not change

- No skill or MCP implementation moves out of its extension.
- No new file watcher, generation, or RPC.
- No generic migration of existing World State sections.
- No change to the stable environment-ID assumption from #29856.

## Example

```text
step 1 snapshot: skills = []
step 2 snapshot: skills = [executor-demo:deploy]

core asks the skills extension to render only that change.
```

## Stack

1. **This PR:** let extensions contribute World State sections.
2. Project executor skills through the skills extension.
3. Pin one MCP runtime to each model step.
4. Project selected MCP/app/connector metadata by environment
availability.
5. One end-to-end integration scenario.

codex-rs/ext/extension-api/src/lib.rs

@@ -38,8 +38,10 @@ pub use contributors::ExtensionFuture;
 pub use contributors::McpServerContribution;
 pub use contributors::McpServerContributionContext;
 pub use contributors::McpServerContributor;
+pub use contributors::PreviousWorldStateSection;
 pub use contributors::PromptFragment;
 pub use contributors::PromptSlot;
+pub use contributors::RenderedWorldStateFragment;
 pub use contributors::ThreadIdleInput;
 pub use contributors::ThreadLifecycleContributor;
 pub use contributors::ThreadResumeInput;
@@ -63,6 +65,8 @@ pub use contributors::TurnItemContributor;
 pub use contributors::TurnLifecycleContributor;
 pub use contributors::TurnStartInput;
 pub use contributors::TurnStopInput;
+pub use contributors::WorldStateContributionInput;
+pub use contributors::WorldStateSectionContribution;
 pub use registry::ExtensionRegistry;
 pub use registry::ExtensionRegistryBuilder;
 pub use registry::empty_extension_registry;