feat(plugin): add session retry hook

[rekram1-node]

Summary

Add a public session.retry plugin hook as the final policy decision for provider failures, without exposing the runner's internal recovery mechanism.

await ctx.session.hook("retry", (event) => {
  if (event.error.status === 429) {
    event.decision = { retry: true, delay: 10_000 }
    return
  }
  if (event.error.type === "provider.invalid-request" && event.attempt === 2) {
    event.decision = { retry: true, delay: 0 }
    return
  }
  if (event.attempt >= 3) event.decision = { retry: false }
})

The hook receives stable Session context (sessionID, agent, model, projected Session error, and the physical attempt being proposed) plus a discriminated decision:

type SessionRetryDecision =
  | { retry: false }
  | { retry: true; delay: number }

• Core initializes decision from its built-in retry classification and backoff. Plugins can veto a proposed retry or force a normally terminal provider failure to retry.
• The hook runs before session.retry.scheduled; the final delay drives both the durable at deadline/UI countdown and actual sleep.
• Hooks run sequentially, so later hooks see and may replace the current decision. Provider scoping uses the event's model reference.
• The initial request is attempt 1 and the first retry is attempt 2, preserving existing durable event semantics.
• The existing five-attempt ceiling is outside plugin control and remains a hard safety bound. Context-overflow compaction remains separate.
• Invalid delays (negative or non-finite) fall back to Core's computed delay.
• The hook does not expose replay/continuation mechanics, so its contract remains valid if recovery implementation changes.

Verification

• Runner scenarios cover vetoing a transient retry, forcing a terminal failure to retry, sequential delay overrides, durable deadlines, and the existing retry ceiling.
• Full isolated Core suite: 3789 pass / 39 skip / 0 fail.
• Core and plugin typechecks pass; plugin tests: 5 pass / 0 fail.
• Documentation Astro check reports 0 errors/warnings/hints; formatting and diff checks pass.