Expose task identity in resolve_context schema and compatibility mapping

[CountableNewt]

Problem

The deterministic resolver requires context.task for task-scoped automatic activation, and standard Agent Skills default to task scope. However, the advertised resolve_context nested context schema omits task while setting additionalProperties: false, and the top-level compatibility mapper also omits task identity. A conforming agent therefore cannot supply the context needed to activate the default standard skill.

Acceptance criteria

• Add optional task identity to the nested structured context schema.
• Add task identity to the one-release top-level compatibility surface and map it into runtime context.
• Keep schemas strict and documented.
• Add contract tests proving the advertised schema and compatibility path preserve task identity.
• Include the fix in Development before source sync/activation and Agent verification.

[linear-code]

MCP-10