System-prompt hash for regression detection

Parent: agent-core

The composed system prompt is SHA-256 hashed; an invariants check builds it 3× and asserts the hash is identical (backend/internal/capabilities/capreg/system_prompt.go — Registry.SystemPromptHash, surfaced as system_prompt_hash by routes/sys/diag_session.go; asserted in e2e registry-invariants.spec.ts). This catches subtle non-determinism — capability assembly-order randomness, map iteration, time-based fragments — that would silently break eval reproducibility.

It rests on a deeper choice: registration order == assembly order (no shuffling, no alphabetic sort), so the prompt is deterministic by construction and the frontend's prompt-part IDs line up.

capreg is being reshaped by

agent-as-injectable-driver — treat these capreg details as current, not final.