Hidden facets: platform-scoped, always-on system-prompt injection

[bdart]

Closes ERMAIN-422. Off main, independent of the client_tools PRs.

What

A new [hidden_facets.facets.<id>] config: baseline system prompts that are always-on, not user-selectable, and never appear in the facet pool — for platform-specific behavior that should wrap the experience as a baseline.

[hidden_facets.facets.outlook_baseline]
platform = "outlook"                 # optional; omit = applies on all platforms
additional_system_prompt = "…"       # static or Langfuse, like regular facets

How

• Injected as a System prompt for requests whose platform matches X-Erato-Platform (GenerationRequestContext.platform), or for a facet declaring no platform.
• Injected once at conversation start, under the same gate as the base system prompt (should_add_system_prompts), then persists in generation_input_messages and replays on later turns. This matches the call's "single injection at the start, not per-turn" conclusion and avoids the accumulation regular facets guard against with toggle detection.
• Distinct AbstractChatSequencePart::HiddenFacetSystemPrompt variant (mirrors FacetAdditionalSystemPrompt); deterministic (id-sorted) order.
• Threaded platform + hidden_facets into compose_prompt_messages / build_abstract_sequence_with_facet_tool_expansions.
• Not added to the /facets response — never selectable.

Open decisions for (originated from — flagged in ERMAIN-422)

1. Single-injection-at-start (implemented) vs re-inject every turn. Implemented matches the call note + the base-system-prompt pattern.
2. platform: Option (None = all platforms, implemented) vs required.

Verification

New test test_hidden_facets_injected_for_matching_platform_only (platform match + all-platforms + non-match). clippy --all-targets --locked -D warnings clean; 45 prompt_composition tests pass; config-reference regenerated; OpenAPI unchanged. Inert until a [hidden_facets] entry is configured.

[hobofan]

I think it makes more sense to instead base this on subkeys on [facets.facets.<facet_id>], like hidden = true and hidden_always_active_for_plaftorm = "outlook".

This way we are not creating another full variant of facets that themselves are special in terms of prompt composition, etc. By re-using the same mainline facet mechanism, we can re-use all of the existing functionality (e.g. also model settings control, etc.)