docs(rules): add pre-dispatch availability and cost checks

subagents.md assumed the Agent tool exists. A new "Pre-Dispatch Checks" section adds two gates before any spawn: Availability (no Agent capability means do the work in the main thread under the same scope and constraints the contract would enforce) and Cost (when writing the full contract costs more than the task, do it inline). Both cross-reference the existing "Don't Subagent At All" guidance rather than duplicating it.

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