refactor: introduce type-safe modelsdev.ID throughout the codebase (Provider.ID(), Store.GetModel, etc.)

[simonferquel-clanker]

Background

During review of PR #2738, several type-safety improvements were proposed and implemented, but ultimately deemed too large to include in the original bugfix PR. This issue tracks them as a follow-up.

Proposed changes

1. modelsdev.ID struct — Add ID{Provider, Model string}, NewID, ParseID, ParseIDOrZero, String() to pkg/modelsdev
2. Store.GetModel(ctx, ID) — Change from bare string to modelsdev.ID, so the compiler prevents passing unqualified model names
3. Provider.ID() modelsdev.ID — Change the provider.Provider interface method to return modelsdev.ID instead of string
4. base.Config.ID() and ModelCapsID() — Return modelsdev.ID
5. modelcaps.Load / LoadFromStore — Accept modelsdev.ID instead of string
6. All downstream callers — getEffectiveModelID, calculateSemanticUsageCost, VectorStore.modelID, EmbeddingConfig.ModelID, rulebased.LastSelectedModelID, team.go, runtime.go, rag/strategy, teamloader, modelinfo, etc.

Motivation

The original bug (#2737) existed because modelcaps.Load took a bare string and callers accidentally passed just the model name (e.g. "claude-sonnet-4-6") instead of the required "provider/model" format. A struct type makes this class of error impossible to compile.

Scope

This is a significant but mechanical refactor — ~50 call sites, mostly renaming string to modelsdev.ID and calling .String() at the boundaries where strings are required (slog, event payloads, etc.).