Let extensions contribute World State sections

[jif-oai]

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

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:

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 Persist selected capability roots and resolve availability per model step #29856.

Example

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.

[chatgpt-codex-connector]

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