feat(prime): analysis of image-model routing (#2453) vs composer model selection and quota tracking

[rynfar]

Background & Upstream Change

Upstream Prime Agent PR #2453 introduced sub-agent image-model routing:

• When sessions encounter image assets or multimodal tasks, sub-agents can be spawned or routed to an alternate, vision-capable model (or a cheaper vision model) instead of strictly inheriting the parent conversation's model.

Analysis for Pylon

1. Interaction with Composer Model Selector

• In Pylon, the user explicitly selects a primary model via the composer selector, or relies on workspace defaults.
• If image-model routing is strictly agent-internal (transparently dispatched by the agent runtime when inspecting images), it would run behind the scenes without overriding the user's primary composer model.
• However, if the user explicitly chose an expensive or specific model, silent fallback to an unconfigured or unapproved provider/model for vision could lead to unexpected behavioral differences or privacy/compliance leaks (e.g. sending images to an external third-party vision provider).

2. Interaction with Quota Tracking & Accounting

• Pylon tracks provider token usage and quotas per instance/workspace.
• If sub-agents route images to a secondary model:
  • Token accounting and spend calculations must be correctly attributed to the secondary model's rates in the journal/ledger.
  • Quota exhaustion on the image model must not wedge the primary conversation or corrupt turn settlement.

3. UI and Configuration Exposure Recommendation

• Phase 1 (Config-only / Agent-internal):
  • Expose an optional setting in provider settings (e.g. imageProcessingModel or visionModel), similar to upstream's auxiliaryModel or subagentDefaultModel.
  • When unset, default to the active session model (preserving existing behavior).
• Phase 2 (UI indicators):
  • In the session inspector / subagent view, indicate when an image-processing subagent ran with an alternate model.
  • Do not complicate the primary composer model picker with auxiliary sub-pickers unless the user enters a specialized multimodal inspection mode.

Action Items

• Evaluate adding an imageModel configuration field to Pylon's Prime provider settings.
• Ensure token usage accounting tracks multi-model costs accurately in session records.
• Define fallback behavior when the configured vision model fails authentication or hits quota.