refactor(runtime): remove the legacy child provider-retry path #3035

Problem

Child provider retries currently retain two admission modes:

• durable_continuation
• legacy_provider_retry

Production Runtime Hosts provide both a SQLite continuation authority and a continuation safety inspector. SessionManager therefore always selects durable_continuation in the supported product composition.

The legacy branch remains reachable only when SessionManager is constructed without either capability. It maintains a second retry planner, replay rules, admission semantics, and tests despite having no production caller.

This parallel path weakens the Runtime Host authority boundary and makes provider-retry changes require reasoning about two implementations.

Proposed change

• Remove legacy_provider_retry from ChildAgentRetryInput.
• Remove planLegacyChildProviderRetry() and fallback-only logic.
• Require continuation authority and safety inspection for child provider retries.
• Fail closed before provider dispatch when the continuation composition is incomplete.
• Preserve provider rate-limit retries through the durable continuation path.

Acceptance criteria

• Child provider retries use only durable continuation admission.
• legacy_provider_retry and its dedicated tests are removed.
• Missing or incomplete continuation authority is rejected before claim or provider dispatch.
• Provider rate-limit retry, continuation recovery, and linked-child behavior remain covered.
• Runtime Host behavior remains unchanged.
• Focused Runtime and Runtime Host tests, type checking, and formatting pass.

Compatibility

Directly constructed SessionManager instances without continuation authority will no longer retry child provider failures. Such compositions are not a supported product execution surface; Runtime Host is the execution authority.

Non-goals

• Changing retry limits or backoff policy.
• Changing root-turn continuation semantics.
• Reworking provider-specific transports.
• Removing fail-closed continuation safety checks.

Context

Verified on main at 1a71db5c0fa85771330354360388ecf47ba95c2e.

Relevant code:

• packages/runtime/src/session-manager.ts
• packages/runtime/src/runtime-kernel.ts
• packages/runtime-host/src/server/execution-composition.ts
• packages/storage/src/sqlite-runtime-store.ts

This issue was identified and prepared with AI assistance (Codex). A human contributor reviewed the analysis and owns the submission.