feat: emit turn_start event carrying provider and model

[akandola]

Closes #725.

What

Implements the accepted spec in #725 (emit provider + model on the wire per turn).

Adds an additive turn_start WireEvent carrying the turn's provider and resolved model, emitted once per turn. Consumers currently cannot attribute a turn's output to a provider/model on protocol v3 — provider and model are internal-only and only surface indirectly via a later provider_switched / provider_error, which fire only on a switch or a failure. turn_start gives observers the provider + concrete model up front, deterministically, on every turn.

| {
    type: "turn_start";
    data: { provider: string; model: string };
  }

Emission points (one turn_start per turn)

turn_start is emitted exactly once per turn. The two emission sites cover the two mutually-exclusive execution paths — a turn runs through one or the other, never both:

• Local / in-process runtime — packages/runtime/src/session/chat.ts (runTurn, via publish). This is the long-lived desktop / self-host runtime (conversation-routes.ts → runTurn → execTurn → backend/session). The model is pin-resolved (resolveModel(pin?.model, pin?.provider)); resolution is best-effort and degrades to blank rather than blocking the turn.
• Cloud per-turn executor — packages/runtime/src/turn/turn-session.ts (runPiTurn, via emit). This is the stateless per-request cloud server (turn/server.ts: one HTTP request = one agent turn). It emits at the first point the concrete model is known, after applying the pin (m.id), so it carries the real model id, not the raw pin.

execTurn (local path) never calls runPiTurn (cloud path); the two are separate code paths, so a given turn produces a single turn_start carrying the concrete model it runs against.

Backward-compatible / additive

• New arm in the WireEvent discriminated union — no existing event is changed.
• Snapshot reducer (packages/runtime-client/src/snapshot.ts) handles the new arm as a metadata marker (grouped with provider_switched / context_compacted): it does not touch running / partial, so live-turn snapshot semantics are unchanged.

Rebase

Rebased onto engine-pod-v0.5.24 (ef469f25), current main. Rebases cleanly — no conflicts. Upstream's chat.ts refactor and this PR's turn_start emission touch non-overlapping regions, so the three-way merge applies them side by side. resolveModel's signature (resolveModel(override?, providerOverride?)) is unchanged across the range, so the local-path call resolveModel(pin?.model, pin?.provider) still matches, and the cloud path still exposes provider + resolved m.id at the emission point.

Tests (all green on ef469f25)

• @houston/protocol typecheck → clean
• packages/protocol/src/wire.test.ts (protocol re-export + discriminant narrowing) → 1/1 pass
• @houston/runtime → 737/737
• @houston/runtime-client → 136/136

Files

• packages/protocol/src/wire.ts — new turn_start union arm
• packages/protocol/src/wire.test.ts — type test
• packages/runtime/src/session/chat.ts — local-runtime emission
• packages/runtime/src/turn/turn-session.ts — cloud-executor emission
• packages/runtime-client/src/snapshot.ts — reducer handles the new arm