fix(agent-core): cancel retry backoff

[drewstone]

What

• add RetryConfig.signal
• stop before a cancelled attempt and interrupt pending backoff
• preserve the exact abort reason

Why

Runtime is consolidating Router retry behavior on agent-core. Without cancellable backoff, cancelling a run can still wait and launch another attempt.

Proof

• pnpm --filter @tangle-network/agent-core test — 421/421 passed
• pnpm --filter @tangle-network/agent-core check-types — passed
• pnpm -r build — passed
• pnpm check:package-artifacts — 9 packages, 19 exports, 38 targets

[tangletools]

✅ Auto-approved drewstone PR — e6d3928f

This PR was opened by the trusted drewstone account.
The full PR reviewer audit still runs separately and will publish findings if it detects issues.

_{tangletools · auto-approval · reason: drewstone_author · 2026-08-04T05:18:22Z}