feat: support native deferred function discovery through Runtime

[SaladDay]

Deferred application functions were rejected at Session admission. This adds shared Runtime search intent and per-function deferral, with Claude's native ToolSearch implementing discovery. Eager functions remain eager; callbacks, result receipts, cancellation and owned native-history recovery reuse the existing path. Core contains no native search or provider logic.

The qualified scope is single-agent Claude environment:none, object-root functions, text results and existing text/inline image messages. Saved/frozen configuration retains tool_search; Session resources follow the pinned response union, which excludes that input member. Exact hosted echo semantics and opaque native-policy fallback detection remain explicit gaps. Other harnesses and workspace/MCP/Subagent/structured-output combinations are not qualified.

Validation: fixed OpenAI SDK 3.13.0 and raw HTTP through PostgreSQL, Core, Docker daemon and pinned Claude SDK/native with real Kimi K3; mixed eager/deferred calls, result retries, SSE/Items, tenant isolation, cancellation, whole-daemon cold continuation and initial/active image input. Native request capture confirms deferred schema loading. Shared Go race tests, 132 Claude tests, server make -o check-web check, local make check-web (73 browser cases), and make openapi passed. No database query changes. Secret scans found no exposure.

Documentation records the protocol limits and milestone-based delivery workflow. A fresh GPT-6 Astra high reviewer independently checked the complete diff and evidence, reran six Go test groups, and found no actionable findings.

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