feat(kernel): retry-with-backoff on retryable provider errors

When the upstream LLM API returns a retryable error (HTTP 429 / 5xx
"overloaded"), the kernel now retries provider.stream() with a stepped
backoff, visibly, until the 8h cumulative-sleep budget is exhausted — then
emits the final error and seals the turn. Retries fire only when no content
was emitted yet this step (safety invariant: never duplicate partial output).

- wire: new transient TurnProviderRetryEvent AgentEvent variant (emitted
  before each sleep; not persisted to model history).
- kernel contracts: RetryStrategy (pure delayFor + injected sleep) + optional
  retry? on RunTurnInput (omit = no retry, backward-compatible).
- kernel run-turn: retry loop in executeStep; providerRetryEvent constructor.
  Kernel imports no timer (sleep injected).
- session-orchestrator: concrete schedule (5s..30m, repeat 30m, 8h budget) +
  abortable setTimeout sleep, wired into RunTurnInput.retry.

tsc -b EXIT 0; biome clean; 1574 vitest pass (+16 new: 11 kernel retry tests
with injected fake sleep + pure delayFor, zero @dispatch/* mocks; 5 schedule
tests). Transports unchanged (transport-ws forwards AgentEvent verbatim in
chat.delta; transport-http is generic JSON.stringify).

Plan: notes/retry-with-backoff-plan.md. tasks.md updated with milestone +
optional CLI-renderer roadmap follow-up.

1 files changed, 14 insertions, 0 deletions

diff --git a/packages/kernel/src/runtime/events.ts b/packages/kernel/src/runtime/events.ts
index b194577..5805e28 100644
--- a/packages/kernel/src/runtime/events.ts
+++ b/packages/kernel/src/runtime/events.ts
@@ -164,3 +164,17 @@ export function errorEvent(
 	}
 	return { type: "error", conversationId, turnId, message };
 }
+
+export function providerRetryEvent(
+	conversationId: string,
+	turnId: string,
+	attempt: number,
+	delayMs: number,
+	message: string,
+	code?: string,
+): AgentEvent {
+	if (code !== undefined) {
+		return { type: "provider-retry", conversationId, turnId, attempt, delayMs, message, code };
+	}
+	return { type: "provider-retry", conversationId, turnId, attempt, delayMs, message };
+}