Centralize model ID configuration

Why

Model IDs are currently hardcoded in three independent places, with no shared symbol or single source of truth:

Failure mode

When the LLM gateway adds, renames, or removes a model, all three sites need updating by people who may not know the others exist. This is especially risky for the triage path — a silent 404 there triggers the fail-closed path, which over-blocks user runs.

The format also drifts: triage uses the fully-dated SKU (`claude-haiku-4-5-20251001`); the others use bare IDs (`claude-sonnet-4-6`). No convention is enforced.

Proposal

Add `src/lib/agent/models.ts` (or similar leaf module) that exports named constants:

```ts
export const AGENT_DEFAULT_MODEL = 'claude-sonnet-4-6';
export const AGENT_PREMIUM_MODEL = 'claude-opus-4-6';
export const MCP_STREAMING_MODEL = 'claude-sonnet-4-6';
export const TRIAGE_MODEL = 'claude-haiku-4-5-20251001';
```

Update all three call sites to import from here. Future model decisions land in one file.

Out of scope

• Gateway capability detection (whether a model is routed)
• Per-program model overrides

🤖 Generated with Claude Code

Start by reading the three listed call sites: src/lib/agent/agent-interface.ts, src/lib/agent/mcp-prompt-streaming.ts, and src/lib/agent/triage-provider.ts. Add the named model constants in src/lib/agent/models.ts or a similar leaf module, update each call site to import them, and run the project’s existing checks to confirm behavior is unchanged.