refactor(agent-test): share the local adapter server

Description

The pitch.

## Problem

The managed-runtime path needs the same strict authenticated loopback server as the public reference adapter without duplicating or drifting its wire behavior.

## Change

- extract the hardened local adapter server into `nest-core`
- reuse it from the public reference adapter
- preserve the version 1 HTTP contract and transactional event semantics
- harden retries, worker ownership, shutdown, and sanitization boundaries

## Verification

- final stack: `make ci-local` — all 5 checks passed; 2,081 passed, 2 skipped, 1 deselected
- extraction/reference parity, replay, shutdown, concurrency, and exception-canary tests pass
- independent review found no Critical or Important issues

## Stack

**6 of 9. Depends on PR 5.** PR 7 adds the OpenClaw connector on top of this shared server.

**Merge note:** After #224 merges into `main`, retarget this PR to `main`, wait for its CI, and only then merge it.