Confidence-based suggestion suppression

[Jam-Cai]

Problem

Tabby currently shows every non-empty suggestion the model generates, regardless of how confident the model is in its output. Low-confidence suggestions tend to be low-quality — vague, repetitive, or contextually wrong — and showing them erodes user trust.

Goal

Add a configurable probability threshold that suppresses suggestions when the model's first-token confidence is too low. This gives Tabby a lightweight quality gate that works across models without prompt changes.

Proposed Scope

• After sampling the first token, inspect its probability (softmax over logits).
• If the top-1 probability is below a configurable threshold, abort the generation and suppress the suggestion silently.
• Expose a settings toggle (default on) and a threshold slider or preset.
• Log suppression events in debug mode with the token, its probability, and the threshold.

Acceptance Criteria

• First-token probability is measurable after sampling.
• Suggestions below the threshold are silently suppressed (no ghost text shown).
• Threshold is configurable with a sensible default.
• Debug mode logs which token was suppressed and its probability.
• Settings toggle, default on.
• No regression in suggestion latency when the gate does not fire.

Relationship to Other Work

• Sibling to Logit gating on first generated token #24 (logit gating on first token) — both live under the Quality epic.
• Logit gating on first generated token #24 is a hard blocklist ("never start with these tokens"). This is a soft quality threshold ("don't show suggestions the model is uncertain about").
• Both features use adjacent sampling infrastructure in LlamaRuntimeCore.
• Applies to the Open Source engine only — Apple Intelligence does not expose token probabilities.

Open Questions

• What is a good default threshold? Needs experimentation across models.
• Should the threshold adapt per model, or is one global value sufficient?
• Should suppression count toward a "model is struggling" heuristic that backs off debounce timing?

Parent: #13