[Feature]: Discoverable thread-title prompt setting

[michidk]

Problem

T3 Code generates thread titles using built-in prompts, but users cannot discover or customize that behavior in Settings. Different users need different title styles—for example shorter titles, ticket IDs, a preferred language, or organization-specific terminology.

The current source defines threadTitleInstructions in TextGenerationPolicy, and the prompt builder supports appending it. However, there is no settings UI for it, and the initial-title and regeneration call sites do not pass a policy, so the capability is not currently usable.

Proposed solution

Add a discoverable Thread title generation setting that lets users provide custom instructions for automatic titles.

The smallest useful scope would be:

• A global custom-instructions field in Settings
• Apply it to both initial title generation and Regenerate title
• A clear Reset to default action
• Help text explaining that the instructions are combined with T3 Code’s built-in formatting and JSON-output requirements

Per-project overrides could be added later.

Discoverability

This should be visible alongside the existing text-generation/source-control writing settings rather than requiring users to know about an internal policy field or configuration file.

Relationship to #6836

Discussion #6836 proposes learning title style from manual renames and retaining rename history. This request is intentionally narrower: it does not require learning, history storage, or examples inferred from prior threads. It asks for an explicit, discoverable configuration surface for instructions that users write themselves.

#6836

Why this matters

Thread titles are navigation metadata that users scan repeatedly. A visible configuration makes title behavior predictable and gives users direct control without modifying T3 Code source.