Contract and snapshot tests — T3, T6

Every public interface named in design.md has an API snapshot under tests/contract/snapshots/ (entities, hooks and contexts, registry, errors, plugins, prompts, skills, models and tools, channels, persistence, configuration and the Settings JSON schema); the agent card and the extension schemas under docs/a2a/ext/ (task, channel, the twelve intrinsic tools) are pinned by snapshot. A snapshot changes only through UPDATE_API_SNAPSHOTS=1 on purpose: Layer 9 recorded two new optional settings (temporal.search_attributes, o11y.trace_file) that way.

## uv run pytest tests/contract -q
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29 passed in 3.08s

## uv run pytest tests/contract -q -k snapshot
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14 passed, 15 deselected in 2.97s