docs: hook-to-subagent research spike ADR (DOJ-2431)

* docs: ADR for hook-to-subagent communication pipeline

Research spike evaluating approaches for triggering the sensei
subagent from hooks without polluting main conversation context.

Closes DOJ-2431

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: tighten ADR guidance for hybrid queueing

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: bitfalt <75431447+bitfalt@users.noreply.github.com>

Author: 3 people