Improvement: Consolidate system prompt prefix and suffix into one field

[yannickmonney]

Problem

Governance “system prompt” is split into a mandatory prefix and mandatory suffix. Authors think in one wrap-around policy text; two fields add friction without a clear product win.

Current behaviour

• Schema: systemPromptConfigSchema with mandatoryPrefixPrompt + mandatorySuffixPrompt (lib/shared/schemas/governance.ts, policy system_prompt).
• Builtin / fixtures: builtin-configs/governance/system-prompt.json, e2e docs-demo fixture.
• UI: SystemPromptEditor — two FormSection + textareas; copy keys governance.systemPrompt.prefixLabel|suffixLabel|….
• Runtime: fetchGovernanceSystemPrompt → prepend prefix / append suffix around agent instructions (governance_prompt.ts, internal_actions.ts, openai-compat path).
• E2E: governance.spec.ts (systemPromptPrefixField).

Proposed change

• Consolidate to a single system-prompt field in schema, org config, and the governance editor.
• Define apply semantics once (e.g. entire string prepended, or a documented wrap convention) and migrate existing prefix+suffix pairs into that field without dropping content (versioned org-config / data migration as required by migrations:check).
• Update i18n, docs, and e2e selectors.

Acceptance criteria

• Governance UI shows one system-prompt field; prefix/suffix controls are gone.
• Existing orgs keep equivalent effective prompt text after migration (prefix+suffix content preserved).
• Chat / openai-compat paths apply the single field consistently.
• Locales, builtin config, fixtures, and e2e updated; migrations:check / config gates green.

Affected service

Platform (web app)