fix: recover Kiro intents after quota exhaustion and lost sessions

[JWThewes]

An existing Kiro intent can remain stuck after a credit outage even when its key has been replaced: a parked stage may lack a saved session, and an answer arriving before callback binding can be mistaken for a conflicting callback owner.

This change:

• Handles answers that race engine/stage callback binding, while retaining callback ownership checks and fencing retired engine runs.
• Rejects Kiro parks without a captured session and successful configured store persistence. A legacy parked stage with matching gate ownership can start a fresh conversation using its saved context and answer; reset, sibling and expired waits remain protected.
• Classifies explicit credit exhaustion separately from invalid credentials and transient throttling, with guidance and a settings link for the intent's pinned credential scope. Regression coverage verifies that rotating that scope takes effect in the same intent.
• Uses consistent recovery and unit-plan reads, surfaces storage failures separately from absent state, and documents export, stage retry and the limits of parallel repair.

Validation: 1,984 backend tests across the agentcore, v2-orchestrator, shared and intents projects pass on Node 24, including durable-runner race/replay and real Git integration tests. All 56 intent-page tests pass, as do frontend typecheck, staged formatting/lint/secret checks and the intents/orchestrator bundles. The commit hooks also pass (959 affected tests; both production dependency audits report zero vulnerabilities). Rebased without changing this patch onto main's separate checkpoint IAM fix #465; its regression test passes.

The reported production sequence is not verified because deployment state was unavailable. A genuinely mismatched approved unit plan still requires operator reconciliation. Provider balance alerts, in-flight configuration refresh and complete diagnostic backups remain separate follow-ups.

Fixes #463.