Phase 38: Universal API skeleton — /v1/chat, /v1/usage, /v1/sessions

First slice of the control-plane pivot. OpenAI-compatible surface
over the existing aibridge → Ollama path. Additive — no existing
routes touched. All 7 unit tests green, release build clean.

What ships:
- crates/gateway/src/v1/mod.rs — router, V1State (ai_client + Usage
  counter), ChatRequest/ChatResponse/Message/UsageBlock types, handlers
  for /chat, /usage, /sessions. OpenAI-compatible field shapes:
  {model, messages[{role,content}], temperature?, max_tokens?, stream?}
- crates/gateway/src/v1/ollama.rs — shape adapter. Flattens messages
  into (system, prompt), calls aibridge.generate, unwraps response
  back into OpenAI /v1/chat shape. Prefers sidecar-reported tokens;
  falls back to chars/4 ceiling estimate matching Phase 21 convention.
- crates/gateway/src/main.rs — one new mod, one .nest("/v1", ...)

Tests (7/7):
- chat_request_parses_openai_shape
- chat_request_accepts_minimal
- usage_counter_default_is_zero
- flatten_separates_system_from_turns
- flatten_concatenates_multiple_system_messages
- flatten_with_no_system_returns_empty_system
- estimate_tokens_chars_div_4_ceiling

Not in this phase (per CONTROL_PLANE_PRD.md): streaming, tool calls,
session state, multi-provider, fallback chain, cost gating. All
land in Phases 39-44.

Next: live-test POST /v1/chat after gateway restart, then migrate
bot/propose.ts off direct sidecar calls to prove the loop end-to-end.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/gateway/src/v1/mod.rs

@@ -0,0 +1,184 @@
+//! Phase 38 — Universal API skeleton (`/v1/*`).
+//!
+//! OpenAI-compatible shape on top of the existing aibridge → Ollama
+//! path. This is the thin slice: single provider, stateless, no
+//! streaming. Phase 39 replaces the direct Ollama call with a
+//! `ProviderAdapter` trait dispatch; Phase 40 adds routing + fallback.
+//!
+//! The shape matches OpenAI's `/v1/chat/completions` closely enough
+//! that clients using openai-compatible SDKs can point at us with the
+//! URL swap alone. We keep the endpoint path `/v1/chat` (not
+//! `/v1/chat/completions`) because our PRD declares the terser form;
+//! adding the alias is one line in Phase 39 when it matters.
+
+pub mod ollama;
+
+use axum::{
+    Router,
+    extract::State,
+    http::StatusCode,
+    response::IntoResponse,
+    routing::{get, post},
+    Json,
+};
+use serde::{Deserialize, Serialize};
+use std::sync::Arc;
+use tokio::sync::RwLock;
+
+#[derive(Clone)]
+pub struct V1State {
+    pub ai_client: aibridge::client::AiClient,
+    pub usage: Arc<RwLock<Usage>>,
+}
+
+#[derive(Default, Clone, Serialize)]
+pub struct Usage {
+    pub requests: u64,
+    pub prompt_tokens: u64,
+    pub completion_tokens: u64,
+    pub total_tokens: u64,
+}
+
+pub fn router(state: V1State) -> Router {
+    Router::new()
+        .route("/chat", post(chat))
+        .route("/usage", get(usage))
+        .route("/sessions", get(sessions))
+        .with_state(state)
+}
+
+// -- Shared types (OpenAI-compatible) --
+
+#[derive(Serialize, Deserialize, Clone, Debug)]
+pub struct Message {
+    pub role: String,
+    pub content: String,
+}
+
+#[derive(Deserialize, Debug)]
+pub struct ChatRequest {
+    pub model: String,
+    pub messages: Vec<Message>,
+    #[serde(default)]
+    pub temperature: Option<f64>,
+    #[serde(default)]
+    pub max_tokens: Option<u32>,
+    /// Accepted for shape-compat but ignored in the thin slice —
+    /// Phase 38 returns non-streaming even when the client asked for it.
+    /// Phase 39+ wires real streaming.
+    #[serde(default)]
+    pub stream: Option<bool>,
+}
+
+#[derive(Serialize)]
+pub struct ChatResponse {
+    pub id: String,
+    pub object: &'static str,
+    pub created: i64,
+    pub model: String,
+    pub choices: Vec<Choice>,
+    pub usage: UsageBlock,
+}
+
+#[derive(Serialize)]
+pub struct Choice {
+    pub index: u32,
+    pub message: Message,
+    pub finish_reason: String,
+}
+
+#[derive(Serialize, Deserialize, Clone)]
+pub struct UsageBlock {
+    pub prompt_tokens: u32,
+    pub completion_tokens: u32,
+    pub total_tokens: u32,
+}
+
+// -- Handlers --
+
+async fn chat(
+    State(state): State<V1State>,
+    Json(req): Json<ChatRequest>,
+) -> Result<Json<ChatResponse>, (StatusCode, String)> {
+    if req.messages.is_empty() {
+        return Err((StatusCode::BAD_REQUEST, "messages must be non-empty".into()));
+    }
+    if req.stream.unwrap_or(false) {
+        tracing::warn!("/v1/chat: stream=true requested but Phase 38 returns non-streaming");
+    }
+
+    let resp = ollama::chat(&state.ai_client, &req)
+        .await
+        .map_err(|e| (StatusCode::BAD_GATEWAY, format!("provider: {e}")))?;
+
+    {
+        let mut u = state.usage.write().await;
+        u.requests += 1;
+        u.prompt_tokens += resp.usage.prompt_tokens as u64;
+        u.completion_tokens += resp.usage.completion_tokens as u64;
+        u.total_tokens += resp.usage.total_tokens as u64;
+    }
+
+    Ok(Json(resp))
+}
+
+async fn usage(State(state): State<V1State>) -> impl IntoResponse {
+    let snapshot = state.usage.read().await.clone();
+    Json(snapshot)
+}
+
+// Phase 38 is stateless — no session persistence yet. Return an empty
+// list in OpenAI-ish shape so clients that probe this endpoint don't
+// 404. Real session state lands in Phase 41 with the profile-system
+// expansion.
+async fn sessions() -> impl IntoResponse {
+    Json(serde_json::json!({
+        "data": [],
+        "object": "list",
+        "note": "Phase 38: stateless. Session state lands in Phase 41.",
+    }))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn chat_request_parses_openai_shape() {
+        let raw = r#"{
+            "model": "qwen3.5:latest",
+            "messages": [
+                {"role": "system", "content": "You are helpful."},
+                {"role": "user", "content": "Hi"}
+            ],
+            "temperature": 0.2,
+            "max_tokens": 100
+        }"#;
+        let r: ChatRequest = serde_json::from_str(raw).unwrap();
+        assert_eq!(r.model, "qwen3.5:latest");
+        assert_eq!(r.messages.len(), 2);
+        assert_eq!(r.messages[0].role, "system");
+        assert_eq!(r.messages[1].content, "Hi");
+        assert_eq!(r.temperature, Some(0.2));
+        assert_eq!(r.max_tokens, Some(100));
+    }
+
+    #[test]
+    fn chat_request_accepts_minimal() {
+        let raw = r#"{
+            "model": "any",
+            "messages": [{"role": "user", "content": "hi"}]
+        }"#;
+        let r: ChatRequest = serde_json::from_str(raw).unwrap();
+        assert_eq!(r.temperature, None);
+        assert_eq!(r.max_tokens, None);
+        assert_eq!(r.stream, None);
+    }
+
+    #[test]
+    fn usage_counter_default_is_zero() {
+        let u = Usage::default();
+        assert_eq!(u.requests, 0);
+        assert_eq!(u.total_tokens, 0);
+    }
+}