Add per-activity instructions field with UI and reset-to-default
Adds configurable detection/level instructions to each activity,
rendered inline in the LLM prompt. Includes settings UI for viewing
and editing instructions, API wiring, sparse storage with reset-to-
default (empty string reverts predefined activities to defaults),
and test coverage.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>