feat(coder): Phase 1-UX backend — agent attribution + agent-sessions route + opencode usage

pending_changes.agent stamped at every queue site (native -> 'boocode', dispatched external -> task.agent, manual RightRail -> NULL) + flows through listPending. New GET /api/sessions/:id/agent-sessions -> [{agent,status,has_session,last_active_at}] per (chat,agent). opencode warm server consumes session.next.step.ended, accumulating input_tokens/output_tokens/cost onto agent_sessions (new idempotent columns) via a pure opencode-usage.ts mapper. Tests: agent-sessions.routes (3) + opencode-usage (6); tsc clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/coder/src/services/backends/opencode-usage.ts

@@ -0,0 +1,77 @@
+/**
+ * v2.6 Phase 1-UX (U.6) — pure mapper for opencode's per-step usage event.
+ *
+ * opencode's warm server emits `session.next.step.ended` once per completed LLM
+ * step (so a multi-tool turn fires it several times). Its `properties` carry the
+ * step's token + cost accounting:
+ *
+ *   {
+ *     timestamp: number;
+ *     sessionID: string;
+ *     finish: string;
+ *     cost: number;                                  // USD for this step
+ *     tokens: {
+ *       input: number; output: number; reasoning: number;
+ *       cache: { read: number; write: number };
+ *     };
+ *     snapshot?: string;
+ *   }
+ *
+ * (Verified against @opencode-ai/sdk@1.15.12 — `EventSessionNextStepEnded` in
+ * `dist/v2/gen/types.gen.d.ts`, a member of the `Event` union the SSE loop
+ * switches on.)
+ *
+ * We normalize to the review's target slice `{input, output, cost}` (the
+ * provider-agnostic `AgentUsage` shape lands later). cache read/write tokens are
+ * folded into `input` so the persisted input count reflects the real context the
+ * model billed for; reasoning tokens are folded into `output` since that's what
+ * the provider counts them as for generation. This keeps the persisted totals a
+ * faithful sum of what opencode reported, without inventing extra columns yet.
+ */
+
+/** The `properties` shape of a `session.next.step.ended` event (subset we read). */
+export interface StepEndedProps {
+  cost: number;
+  tokens: {
+    input: number;
+    output: number;
+    reasoning: number;
+    cache: { read: number; write: number };
+  };
+}
+
+/** Normalized per-step usage delta persisted onto the agent_sessions row. */
+export interface StepUsage {
+  input: number;
+  output: number;
+  cost: number;
+}
+
+/** Coerce a possibly-missing/NaN number to a non-negative finite integer (tokens). */
+function n(v: unknown): number {
+  const x = typeof v === 'number' ? v : Number(v);
+  return Number.isFinite(x) && x > 0 ? Math.round(x) : 0;
+}
+
+/** Coerce a possibly-missing/NaN number to a non-negative finite float (cost USD). */
+function f(v: unknown): number {
+  const x = typeof v === 'number' ? v : Number(v);
+  return Number.isFinite(x) && x > 0 ? x : 0;
+}
+
+/**
+ * Map a `session.next.step.ended` payload → the normalized `{input, output, cost}`
+ * delta. Defensive against missing/partial token blocks (the wire is trusted but
+ * we never want a NaN to poison the accumulated DB total). `input` folds in cache
+ * read+write; `output` folds in reasoning.
+ */
+export function stepEndedToUsage(props: Partial<StepEndedProps> | undefined): StepUsage {
+  const t = props?.tokens;
+  const cacheRead = n(t?.cache?.read);
+  const cacheWrite = n(t?.cache?.write);
+  return {
+    input: n(t?.input) + cacheRead + cacheWrite,
+    output: n(t?.output) + n(t?.reasoning),
+    cost: f(props?.cost),
+  };
+}