Add an optional systemPromptOverride to spawnAgentParams. When non-empty,
the spawned child runs under that persona system prompt instead of the
generic child base prompt (executor instructions still appended), so a
caller can spawn a specific persona — the primitive the compound-
engineering reviewer/research fan-out needs.
Two spikes confirmed the need: fn_spawn_agent had no persona param, and
Fusion has no plugin agent-contribution channel — so the lightweight path
is a generic override here + plugin-local persona defs the skill reads and
passes inline (revised KTD-4/U2/U3 in the plan). Behavioral coverage lands
with U10.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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